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The testis of Nucella consists of numerous tubules, all directed inwards and joining to form a common testicular duct. In a single tubule the spermatogonia lie round the periphery. Mature sperm line the lumen of the tubule. Cells in the same stage of spermatogenesis are grouped together and all members of a group pass through spermatogenesis in phase. Staining with fast green before and after treatment with Van Slyke reagent indicates a change from lysine-rich to arginine-rich histone in the maturing spermatid. Sperm of Nucella are motile throughout their length. The sperm are thread-like and about 80 μ long. The head is Feulgen-positive and about 40 μ long. The mid-piece lies behind the head and is about 8 μ long. The flagellum runs from the front end of the head to the tip of the tail; in the head it is completely surrounded by the nucleus. The spermatogonia contain two centrioles situated near the nucleus and a conspicuous Golgi complex. There are synaptinemal complexes in spermatocyte nuclei in the synapsis stage. In the early spermatid the centriole pushes a tube through the nucleus. This tube is lined by nuclear membrane and is occupied by the anterior portion of the flagellar shaft. The nucleus elongates and the nucleoprotein condenses into strands arranged helically along the long axis of the nucleus. These strands fuse to form lamellae, which disappear in the mature sperm. Mitochondria aggregate at the base of the early spermatid nucleus and form a loose spiral around the flagellar shaft. The outer mitochondrial membranes fuse. The mid-piece of the mature sperm consists of a large tubular mitochondrion enclosing a portion of the flagellar shaft. At the early spermatid stage a pro-acrosomal granule is formed from a large Golgi complex. From this the acrosome develops; it consists of a cone and an acrosome granule. There are two sets of microtubules associated with the acrosome, one lying within the cone, the other outside the cone and separated from it by a ‘ragged membrane’. The microtubules of the outer set extend backwards along the head for two-thirds of its length. The centriole which gives rise to the flagellar shaft lies at the anterior end of the head and is separated from the acrosome by a thin layer of nucleoprotein and a double layer of nuclear envelope. There is no second centriole or derivative thereof in the mature sperm. In the tail groups of coiled fibres are associated with each pair of the peripheral flagellar fibrils.

Nucleoli in some large yolky oocytes of Plethodon cinerens and Ascaphus truei have attached to them a filament which may or may not have a beaded appearance. These filaments have been called ‘nucleolar tails’. One nucleus may contain several hundred of them. They may be up to 200 μm long and are usually about 1 μm wide. The beads which are sometimes attached to the filament are round, refractile and up to 3 μm in diameter. The tails are a feature of nuclei in which the nucleoli have left the nuclear envelope and migrated inward to cluster around the chromosomes in the centre of the nucleus. The tails project radially outwards from the central mass of nucleoli. Tails may be attached to round solid nucleoli, as is usually the case in A. truei, or they may be attached to ring nucleoli, as is often the case in P. cinereus, or they may lie free in the nuclear sap. Those nucleoli which remain attached to the nucleolar organizing site on the lampbrush chromosomes of P. cinereus never have tails. Nucleolar tails are Feulgen-negative and do not stain with gallocyanine when it is used under the proper conditions for staining nucleic acids. Tails are unaffected by deoxyribonuclease and ribonuclease but are destroyed by proteolytic enzymes. Thin sections of oocytes whose nuclei contained nucleolar tails showed chains of membranous vesicles lying near to many of the nucleoli. These vesicles appeared empty, some of them were confluent with their neighbours, all were bounded by a unit membrane and all were coated on their outer surfaces by a fine fibrous material. The chains of vesicles commonly ended near a depression on the surface of the core of a nucleolus. Chains of vesicles sometimes extended into a long membranous tube. We have interpreted these chains of vesicles and tubes as sections through nucleolar tails. It is suggested that nucleolar tails may be associated with the replication of nucleolar DNA, or with the division of nucleolar cores and nucleoli, or with the production of nuclear membrane at a time when the nucleus is enlarging rapidly. Each of these suggestions is discussed. A likeness between nucleolar tails and the mesosomes of bacteria is proposed and discussed.

Amphibian oocytes were incubated in vitro in the presence of [3H]uridine, and autoradiographs were made of nucleoli isolated from these oocytes and of sections of oocytes. After incubations of 2 h or less the nucleoli of oocytes larger than 0·6 mm diameter are asymmetrically labelled. With longer incubations nucleoli from oocytes of 0·6 tori mm diameter become more uniformly labelled. Those of oocytes larger than 1·2 mm diameter remain asymmetrically labelled whatever the incubation time. Autoradiographs of 1-μ sections through oocytes larger than 0·6 mm diameter show, after short incubations, asymmetrically labelled nucleoli. In these autoradiographs silver grains are concentrated over a distinct component of each nucleolus which is eccentrically placed towards the nuclear envelope. Thin sections of oocytes show nucleoli consisting of core and cortex. The core material is always concentrated into the half of the nucleolus which, lies nearer the nuclear envelope. Autoradiographs of separated nucleolar cores and cortices from oocytes larger than 0·6 mm diameter show, after short incubations, silver grains over cores but not over cortices. Similar autoradiographs prepared from oocytes of 0·6 to 1·1 mm diameter, after longer incubations, show grains over cores and cortices. These results appear to indicate that nucleolar RNA is synthesized in the nucleolar core, in association with the nucleolar DNA, and is thence transferred to the cortex where it is built into ribonucleoprotein particles. Initial asymmetrical labelling is a consequence of the eccentric location of the nucleolar core. The nucleoli of oocytes smaller than 0·6 mm diameter always label symmetrically; such nucleoli consist entirely of core material. It is suggested that the nucleoli of oocytes larger than 1·2 mm diameter always label asymmetrically because transfer of RNA from core to cortex proceeds more slowly than in smaller oocytes.

SYNOPSIS. In S. fallax and D. cirratuli the pellicle is thrown into longitudinal folds. The pellicle consists of 2 membranes, one 3-layered, the other 5-layered. Sub-pellicular fibrils, each about 23 mμ in diameter run along the pellicular folds. Pellicle and sub-pellicular fibrils together form the mechanism by which Selenidiids move. The undulating membranes of D. cirratuli are enlarged pellicular folds, the tips of which are packed with fibrils. The light ovoid patches seen in thin sections are sections through regions which contain a form of glycogen in the living animals. Typical mitochondria are absent. Objects described as lipochondria have a regular distribution at the bases of the grooves in the pellicle and are probably formed from the inner components of the pellicle.

Newt oocytes were dissected under liquid paraffin and known volumes of clean nucleoplasm and cytoplasm were obtained from oocytes in different stages of oogenesis. The samples were digested in redistilled nitric acid, diluted with deionized water, and their Na and K contents were measured by flame spectrophotometry. The results were expressed as micro-equivalents of Na and K per millilitre of nucleoplasm or cytoplasm. In oocytes of 0·3—0·5 mm diameter nucleoplasm and cytoplasm have similar Na and K concentrations, and the molar K:Na ratio is about 3:1. As the oocyte grows to maturity the nucleoplasmic Na and K concentrations do not change, but the cytoplasmic K concentration falls steadily until, in nearly mature oocytes, the cytoplasmic K:Na ratio is near 1:1. Measurements of Na and K concentrations in yolky and clear cytoplasm show that both fractions have the same K content and the concentration of K decreases in both fractions as the oocyte grows. The significance of these results is discussed in terms of the possible effects of changes in the intracellular ion balance on the morphology and synthetic activity of the chromosomes.

The dry mass of nucleoli from yolky oocytes of T. cristatus, as determined by interference microscopy is about 150 μμg. Less than 10% of this is attributable to RNA that is removable by ribonuclease from formalin-fixed nucleoli. There is a linear relationship between nucleolar dry mass and size. There are basic proteins in oocyte nucleoli and these are bound to the nucleolar RNA. The concentrations of cytochemically detectable RNA and basic protein are higher in the nucleolar core than in the cortex. The ratio of RNA to basic protein is higher in the core than in the cortex, as determined by microdensitometry of nucleoli stained with gallocyanine and fast green. The significance of these observations in relation to the role of the nucleolus in ribosome biosynthesis is discussed.

The effects of hypophysectomy and of subcutaneous injection of gonadotrophin upon whole oocytes, oocyte nuclei, and lampbrush chromosomes of the newt Triturus cristatus carnifex have been studied. Hypophysectomy leads to a reduced rate of incorporation of 32P into the whole oocytes and oocyte nuclei. Large, yolky oocytes are more severely affected than are small oocytes lacking yolk. Hypophysectomy also leads to decreased stiffness of oocyte nuclear sap, change in the number and character of free granules in the nuclear sap, and alteration in the size of certain objects occurring at specific sites on the lampbrush chromosomes. Injection of mammalian gonadotrophin leads to an increased rate of uptake of 32P into the cytoplasm and nuclei of yolky oocytes. Gonadotrophin treatment also leads to increased stiffness of oocyte nuclear sap, decrease in the number and/or the size of free granules in the nuclear sap, reduction in size of the giant loops of chromosomes X, XI, and XII, and changes at the axial granule loci of chromosome I and the sphere loci of chromosomes V and VIII. Each of the features mentioned above varies in nature from one animal to another. This inconstancy, which is superimposed upon stable genetic characteristics and upon those developmental changes which normally accompany oocyte growth, is now shown to reflect differences in the physiological states of the ovaries of different newts.

It is surprising that the Fourth Evangelist, with his marked interest in the symbolical, should have omitted from his account of the Last Supper all reference to the Eucharist. Yet the instinct of the Church has rightly seen in ‘John’ (whoever be the person behind the name) the supreme New Testament teacher on the sacraments. Time and again, sometimes with hints that are not always obvious save to those who have insight into his mind and methods, he points his readers to the two great sacraments of the Church. It is with good reason that medieval artists are wont to portray Peter carrying the keys, Paul the sword, but John the sacramental cup.

The ovaries of newly metamorphosed Xenopus females contain oocytes in all stages of early meiotic prophase. In pachytene nuclei extrachromosomal nucleolar DNA appears in the form of a thin cap covering one side of the nucleus. During pachytene this cap of DNA enlarges to occupy half the nucleus. After pachytene the nucleus grows rapidly and the cap of DNA disperses into numerous tiny granules which become scattered throughout the nucleoplasm. Autoradiographs of cells which have been incubated with [3H]thymidine, and microspectrophotometric measurements of the Feulgen dye contents of nuclei in various stages of meiosis, show that the extrachromosomal nucleolar DNA is synthesized during the pachytene and immediate post-pachytene stages. Microspectrophotometric comparison of the Feulgen dye contents of post-pachytene nuclei in which the DNA cap has dispersed, and similarly prepared mouse liver nuclei, show that the post-pachytene nuclei have about 30 µµg of extrachromosomal pachytene nucleolar DNA. In [3H]thymidine autoradiographs early pachytene nuclei are less heavily labelled than late pachytene and early diplotene nuclei. Consequently it is proposed that primary replicas of the chromosomal nucleolar organizer undergo a series of post-detachment replications.

I hope that no apology is necessary for the choice of a subject so closely akin to that of the Presidential Address by Professor William Manson last year.1 It was in fact that address which inspired the present paper. If any further justification is necessary for reverting to this theme I would remind you that a presidential address is by use and wont exempt from criticism, so that last year there was no opportunity for the discussion of a subject which should surely invite lively debate. Moreover, though Dr Manson in his address made occasional reference to Paul's thought, he deliberately confined himself in the main to the ‘Spiritual Background of the Work of Jesus in the Synoptic Gospels’. Such promising exploration deserves to be followed up, for it is only with Paul that this background is filled in, in all its tremendous cosmic grandeur. As Dr Manson put it, the spirit world in Paul's thought ‘has taken on new dimensions and acquired a cosmic range and character’. Or again, whereas last year we were meeting the demonic powers ‘in the form of malignant and ghoulish beings, on the earth-level of popular demonic belief’, this year we are confronted by Paul's ‘grandiose hierarchy of cosmic spirits’. But my best justification is the sheer importance of this subject for the understanding of Paul's thought. As Professor J. S. Stewart has remarked in a notable article in the Scottish Journal of Theology (vol. IV, no. 3), we shall never get inside the mind of Paul until we take seriously what has in fact been ‘a neglected emphasis in New Testament Theology’, and cease to treat ‘as secondary and extraneous elements in the primitive Christian proclamation what in fact are integral and basic components of the Gospel’.

Our General Meeting this year is to concern itself with a subject of a highly technical and specialist character. Your President has been expressly absolved from the obligation of selecting a subject related to Qumran. And indeed it may be thought more fitting that a Presidential Address should deal with a subject of a more general interest and with a more immediate bearing upon the life and destiny of mankind.

When DNA from blood or liver of Plethodon c. cinereus is centrifuged to equilibrium in cesium chloride it separates out into 2 components. The smaller or satellite component is relatively rich in G + C and is therefore heavy, and it amounts to about 2% of the total DNA. The heavy satellite does not include the ribosomal cistrons, and it is unrelated to the nucleolar organizer. When squash preparations of cells from the testis of P. c. cinereus are incubated in synthetic E3RNA complementary to the satellite DNA, the RNA anneals specifically to the centromeric heterochromatin of spermatogonia, spermatocytes, and spermatids, and to the centromeric regions of all discernible chromosomes. RNA/DNA hybrids were located by autoradiography. H3RNA complementary to the major component of the DNA anneals to all nuclei and to all parts of the chromosomes. H3RNA complementary to nucleolar DNA from Xenopus laevis anneals specifically to the chromatin associated with nucleoli in nuclei at various stages of the meiotic divisions. The nature of the centromeric heterochromatin and its role in the meiotic divisions are discussed.

The salivary glands of 3rd or 4th instar larvae of Simulium niditifrons are about 5 mm long and up to 400 µ wide. They have a capacious lumen which is normally filled with secretion. The apical (luminal) plasmalemma of the gland cells is thrown into numerous microvilli. The basal plasmalemma is usually straight but is infolded in places. The infoldings may be complex near to cell junctions. There is a thick, uniform basement membrane. Contact surfaces of adjacent cells often interdigitate. A septate junction extends inwards from the lumen for one-quarter the depth of the cells. Rough endoplasmic reticulum is distributed evenly throughout the cytoplasm. Many Golgi complexes with dark membrane-bounded granules are scattered throughout the cytoplasm. Solitary granules, often more than I µ in diameter, lie in the apical cytoplasm, especially near the apical border of the cell. These granules resemble the larger Golgi granules and the contents of the lumen. Solitary granules consisting of 2 components have been seen in various stages of passage through the cell membrane. The 2 components are present in roughly constant proportions and can be identified in the larger Golgi granules and in the secretion in the lumen. The nucleus is spherical. The nuclear envelope is smooth in the larger cells of a gland but may be folded in the smaller cells. There are 80-100 pores/µ2 of nuclear envelope. Each pore appears to have a small granule at its centre. Microtubules, about 180 Å thick, are numerous in the apical cytoplasm, particularly near the luminal border. Tubules which lie deep in the cytoplasm are flanked by a clear area 100-200 Å wide. The fine structure of a salivary gland cell of Simulium appears to indicate that the major components of the salivary secretion are synthesized in association with the ribosomes on the rough endoplasmic reticulum, concentrated in the Golgi regions, formed into secretion granules, and passed out of the cell into the lumen of the gland by reverse phagocytosis.

The chromomeres of lampbrush chromosomes of Triturus cristatus are Feulgen-positive; they therefore contain DNA. After removal of their DNA in boiling trichloracetic acid, the chromomeres stain with fast green at alkaline pH; they therefore contain basic protein. The lateral loops are Feulgen-negative; they stain with toluidine blue at acid pH, but much less intensely following RNase digestion; they therefore contain RNA. The spheres of chromosomes V and VIII do not contain RNA. Unfixed lampbrush chromosomes retain a life-like appearance in 0.07 M K/NaCl at pH 6.2; in this medium the nuclear sap disperses. As pH is raised to 8.5 the matrices of lateral loops dissolve but chromosome axes remain unbroken. Above pH 8.5 lampbrush chromosomes dissolve. As pH is lowered from 6.2, at between 5.8 and 5.4 coagulation occurs. If pH is rapidly reduced still further, a persistent relaxed condition sets in between 2.5 and 2. In concentrations of K/NaCl above 0.5 M lampbrush chromosomes dissolve. Lateral loop matrices dissolve in 0.25 M K/NaCl but chromosome axes remain unbroken. In concentrations of K/NaCl below 0.05 M lateral loop matrices dissolve, but even in distilled water chromosome axes remain unbroken. Trypsin at pH 6.2 and at pH 7.8 strips the matrices from lateral loops and occasionally breaks matrix fusions. It causes chromomeres to swell and coalesce, but fails to break chromosome axes. The action of ‘pan-protease’ resembles that of trypsin in all respects. Pepsin at pH 6.2 strips the matrices from lateral loops, but does not destroy chromomeres. At low pH peptic digestion is slow: the enzyme is attacking coagulated chromosomes; but if peptic digestion precedes a lowering of pH the overall outcome is a rapid solution of loop matrix, and under these conditions matrix and sphere fusions are broken. If trypsin or ‘pan-protease’ digestion precedes a lowering of pH there is a similarly rapid solution of loop matrix; thus the action is not specifically referable to pepsin. Under no conditions does pepsin break the axes of lampbrush chromosomes. RNase at pH 6.2 strips the matrices from lateral loops; this action is detectable at extreme dilution. RNase does not destroy chromomeres, nor does it break chromosome axes. If tryptic digestion follows RNase digestion this too fails to break chromosome axes. Unlike the proteolytic enzymes and RNase, DNase at pH 6.2 breaks the fibril between adjacent chromomeres, and it also breaks the axes of lateral loops. Contrary to Mazia's experience with salivary gland chromosomes, versene does not break the axes of lampbrush chromosomes even when applied in media of low electrolyte concentration. These results indicate that uninterrupted fibres of DNA run throughout the lengths of lampbrush chromosomes.

The main features of lampbrush chromosome organization are reviewed and the significance of RNA transcription on lampbrush loops is questioned. Special consideration is given to evidence for the transcription on lampbrush loops of satellite DNA, low copy number genes with defined functions, the histone genes, the 5S genes and the genes for ribosomal RNA. It is concluded that there is widespread but somewhat indiscriminate transcription on lampbrush loops of a range of repetitive DNA sequences, transcription of a wide range of mRNAs, continuous transcription of histone and 5S RNA, and low or aberrant transcription of ribosomal RNA. The 'read-through' hypothesis of lampbrush loop transcription is explained and evaluated, and some of the assumptions underlying it and the problems it raises are examined and discussed. The hypothesis requires that transcription starts at a normal promoter site for a functional gene situated at the thin end of a lampbrush loop, and that once started the transcribing polymerase cannot stop until it reaches another promoter that is already initiated or some condensed and untranscribable chromatin. The following questions are considered. Why should polymerase on a lampbrush loop disregard normal termination signals? Why should polymerase stop when it encounters another initiated promoter sequence? Why is the number of loops or transcription units related to genome size signifying, according to the read-through hypothesis, that oocytes from animals with large genomes have more active 'functional gene promoters' than those from animals with small genomes? Finally, some special situations where there is enhanced or reduced lampbrush activity are considered and their significance in relation to ideas about the function of lampbrush chromosomes is discussed. Specific examples include the frog, Ascaphus truei, whose oocytes have eight germinal vesicles each with a full complement of lampbrush chromosomes, another frog, Flectonotus pygmaeus, in which each oocyte starts with several thousand meiotic nuclei only some of which go into a lampbrush phase, and certain species of reptile in whose germinal vesicles the chromosomes never acquire a lampbrush form.

Methods are described for in situ hybridization of ribosomal DNA from Xenopus laevis, labelled in vitro with 125iodine, to mitotic and lampbrush chromosomes from Triturus cristatus carnifex. The hybridization reaction was carried out in a mixture containing 50% formamide, 4 X SSC, 0.1 M KI, at 37 degrees C, or in 2 X SSC, 0.1 M KI at 65 degrees C. Autoradiographs of mitotic metaphases from 2 males showed labelling over the middle of the short arm of one chromosome IX in each metaphase. In some cases, a region near the end of a longer chromosome was also labelled. In a lampbrush preparations, labelling was confined to a region identified as about 53 units, near the middle of the short arm of both halves of bivalent IX. The usefulness of the technique and the significance of the labelling of only 1 of the 2 chromosomes IX in mitotic preparations are discussed.

The general features of the arrangement and cytological distribution of repeated sequences in animal chromosomes are reviewed. These features include internal repetitiveness, conservation of clearly functional sequences, rapid divergence of certain classes of repeated sequence with subsequent fixation of families of diverged sequences, and well defined cytological localization of large blocks of sequences in specific parts of the chromosome set. Moderately or ‘middle’ repetitive (m.r.) sequences constitute a large part of the genomes of higher organisms, they seem to accumulate in a balanced manner within chromosome sets, they are mainly responsible for genome growth, and they are interspersed with sequences of other kinds. Little is known about their cytological distribution. Four experiments are described, each of which aimed to locate middle repetitive sequences in the chromosomes of a salamander and a newt. Tritiated m.r. DNA from Plethodon cinereus binds in a non-random fashion to the meiotic diplotene and mitotic chromosomes of the same species, suggesting a non-random distribution of m.r. sequences on these chromosomes. The same DNA, hybridized in situ to the RNA transcripts on the loops of lampbrush chromosomes, produces light and widespread labelling of many loops, but intense labelling of six pairs of loops, each of which lies near to a centromere. Similar experiments in which newt m.r. DNA was hybridized in situ to newt lampbrush chromosomes showed heavy labelling of about 30 loop pairs on each of the long heteromorphic arms of chromosome I, but very little labelling elsewhere. Autoradiographs of newt mitotic chromosomes hybridized with newt m.r. DNA showed rather even labelling of all chromosomes including chromosome I. The significance of the heavy labelling of lampbrush loops near centromeres and on the heteromorphic arms of the newt chromosome I is discussed in relation to possible cytological and molecular mechanisms for generating and preserving families of tandemly linked and cytologically localized m.r. sequences.

The functional and evolutionary significance of highly repetitive, simple sequence (satellite) DNA is analysed by examining available information on the patterns of variation of heterochromatin and cloned satellites among newts (family Salamandridae), and particularly species of the European genus Triturus . This information is used to develop a model linking evolutionary changes in satellite DNAs and chromosome structure. In this model, satellites accumulate initially in large tandem blocks around centromeres of some or all of the chromosomes, mainly by repeated chromosomal exchanges in these regions. Centromeric blocks later become broken up and dispersed by small, random chromosome rearrangements in these regions. They are dispersed first to pericentric locations and then gradually more distally into the chromosome arms and telomeres. Dispersal of a particular satellite is accompanied by changes in sequence structure (for example, base substitutions, deletions, etc.) and a corresponding decrease in its detectability at either the molecular or cytological level. On the basis of this model, observed satellites in newt species may be classified as ‘old ’, ‘young’, or of ‘intermediate’ phylogenetic age. The functions and effects of satellite DNA and heterochromatin at the cellular and organismal levels are also discussed. It is suggested that satellite DNA may have an impact on cell proliferation through the effect of late-replicating satellite-rich heterochromatin on the duration of S-phase of the cell cycle. It is argued that even small alterations in cell cycle time due to changes in heterochromatin am ount may have magnified effects on organismal growth that may be of adaptive significance.

The telotrophic ovary of Notonecta glauca glauca consists of 7 ovarioles. Each ovanole comprises, from front to rear, a terminal filament, a trophic region, a prefolhcular region, and a series of 10-15 follicles of progressively increasing size The trophic region is largely syncytial and is made up of polyploid trophic nuclei packed around a central trophic core The cytoplasm of the trophic core is continuous with the cytoplasm of each oocyte through a system of trophic tubes. There is one trophic tube per oocyte. The trophic nuclei have large nucleoh. There are a few small nucleoh in the oocyte nuclei The cytoplasm of the trophic core, the trophic tubes, and the oocytes is rich in RNA. Autoradiographs of sections of ovarioles fixed 2 h after injection of PHjundine into animals show label over the trophic nuclei only. Eight-hour autoradiographs show heavy labelling of the trophic region and label over the front ends of the trophic tubes, but little label over the posterior regions of the tubes or the oocyte cytoplasm. Later autoradiographs indicate that label gradually spreads backwards from the trophic core, along the trophic tubes, and progressively builds up in the oocyte cytoplasm These observations are thought to indicate synthesis of RNA in the trophic region and movement of RNA from the trophic core along the trophic tubes to the oocytes The trophic core and tubes show brilliant positive form birefringence with respect to their lengths. This birefringence can be reduced by keeping animals at 2 °C for 12 h, and eliminated by placing ovanoles in 1 % colchicine for 6 h. Electron micrographs of thin sections of ovanoles show that trophic core and tubes are densely and uniformly packed with nbosomes and microtubules The latter are lined up along the trophic tubes. There are about 30000 microtubules evident m a TS through a trophic tube 15 fim wide. Lengths of microtubules up to 2 /*m have been observed. Ribosomes are packed between the microtubules but are excluded from regions where the spacing between adjacent microtubules is less than 25 ran The contribution of the trophic region to the oocytes and the role of the microtubules in maintaining or facilitating the movement of nbosomes along the trophic tubes is discussed

DURING the development of the amphibian oocyte, the germinal vesicle becomes enlarged and can be seen to contain very many nucleoli1,2 instead of the four which would be expected from the tetraploidy of the egg nucleus. The RNA of these nucleoli is similar in base composition to ribosomal RNA3 and each nucleolus contains a small amount of DNA4–6. Amphibian oocyte nucleoli can be hypotonically disrupted to reveal DNP rings in the shape of beaded circles or necklaces2,6. There is compelling evidence in amphibia2,6,7 as well as in the insect Tipula8 that this DNA is derived from the chromosomal nucleolar organizer region which contains the ribosomal cistrons9,10. This conclusion is supported by the recent finding that in ovarian tissue of Xenopus there is a disproportionately greater synthesis of ribosomal cistrons11 which leads eventually to a higher proportion of them than that found in DNA of somatic tissues11,12. In Bufo, the nucleolar DNA migrates as granules from the chromosomal bouquet to the periphery of the nucleus where, at later stages, the granules give rise to nucleoli5. A similar process occurs in Xenopus where Feulgen-positive material appears in the oocyte nucleus as a “nuclear cap” at leptotene13. After zygotene this cap disperses and numerous nucleoli are formed which contain one or several DNA cores or rings14.

The oocytes of naturally occurring triploid females of the Ambystoma jeffersonianum complex each contain 84 lampbrush chromosomes. This constitutes hexaploidy (n = 14). The chromosomes are joined into pairs by chiasmata and form 42 bivalents. It is suggested that meiosis in triploid females is preceded by an endomitosis and the resulting sister chromosomes synapse to form pseudo-bivalents. Sperm from diploid males stimulate development of the triploid eggs but do not contribute chromosomes to the triploid nucleus. Bivalents in the oocytes of triploids have twice as many chiasmata as the corresponding bivalents in diploid animals. Such chiasmata cannot result in genetic recombination.

Male meiosis, with special regard to the centromeric heterochromatin and to centromeric structure, has been studied in the salamander, Plethodon cinereus cinereus. In this salamander, n = 14. Early meiotic prophase proceeds as described by other authors. Pachytene is followed by a “diffuse” stage in which much of the chromosomal DNA becomes reorganized into fine lateral loops which spring from the bivalent axes. These loops can be seen along the bivalent axes as early as zygotene. Loops are maximally extended in the diffuse stage. The formation of diplotene bivalents involves a return of this extended DNA into the axes of the bivalents. — At leptotone, centromeric heterochromatin is in one or a few large masses. These masses break up during zygotene. At pachytene there is one mass of heterochromatin at the centromeric region of each bivalent. The heterochromatin remains condensed in the diffuse stage. During diplotene, centromeric heterochromatin becomes less conspicuous, and it is possible to see 4 centromere granules in each diplotene bivalent. These observations support the view that centromeres replicate at pre-meiotic S-phase when the associated hetero-chromatin is replicated. In the interphase before the 2nd division, the hetero-chromatin often forms a broken ring corresponding to the positions of the centromeres at the end of anaphase 1. There are 14 masses of heterochromatin in nuclei at prophase of the 2nd division. In spermatids, the heterochromatin appears as a single solid mass or a broken ring.

UNTIL a few years ago it was generally assumed that nucleic acids lack the diversity required of primary genetic material, and proteins alone were supposed to possess this attribute. The results of early studies of enzymic digestion of chromosomes1 appeared to support this concept, for although nucleases and pepsin were found to be unable to break chromosomes, trypsin effected complete disintegration. Darlington's theory2 of the nucleic acid ‘charging’ of chromosomes is based on the supposition that the linear integrity and genetic specificity of chromosomes is maintained by a backbone of protein, to which deoxyribonucleic acid becomes ‘attached’ at certain stages in mitotic and meiotic cycles. If such deoxyribonucleic acid acquires specificity, this is derived through association with chromosome protein.