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The present study was undertaken in order to determine whether the behaviour of the primordial gonocytes of chick embryo in reaching the genital primordia is in agreement with the chemotactic theory of their migration. An additional aim was the demonstration of a possible excretory function of the germinal epithelia. Ultrastructural observation provides evidence for amoeboid and phagocytic properties of chick germinal cells (figs 1–12). The penetration of primordial gonocytes into the genital primordia is an active process: their autonomous, directed progression is accompanied by intense lytic and phagocytic activity. The gonocytes ingest soluble products (pinocytosis) and cellular debris (phagocytosis) emitted by the attracting germinal epithelia. The affinity of the gonocytes for the gonadal primordia agrees with the idea that their migration is determined by a positive chemotaxis. The germinal epithelia have a syncitial, dissociated appearance (figs 13–15). This disaggregated aspect is due, not only to the lytic activity of gonocytes which become established in the gonadal primordia, but also to intrinsic excretory properties. The ultrastructural description of the young germinal epithelium (figs 16–27) has been combined with autoradiographic study, at high resolution, of the migration of radioactivity incorporated into newly synthesised proteins ( 3 H-leucine; figs 16–27). The results show that the epithelial cells of the gonadal primordia excrete exportable proteins by means of swellings which burst at their surface. The young germinal epithelium of chick embryo should therefore be regarded as a secretory organ which expels its elaborated products (proteins in this instance) by an excretory process of the merocrine type.

The turnover of nuclear and cytoplasmic proteins in the attractive germinal epithelia of the embryonic chick has been studied by kinetic analysis combined with quantitative electron microscope autoradiography. The present report consists of a mathematical treatment of numerical data obtained by two series of conceptually different experiments: In the first series, involving only cultured blastoderms, a 15-min incorporation of 3H-leucine was followed by a 10-h chase. In the second series, a long incorporation (3 h) of radioactive precursor by cultured blastoderms was followed by a 2-h chase involving only the isolated gonadal region in organotypic culture. Mathematical analysis of the decay of radioactive protein in the two series shows that the results are in very close agreement, and provides evidence for the existence of kinetically different processes in the nucleus and in the cytoplasm. Despite certain imperfections inherent in the methods which have been employed, and the complexity of the biological system under consideration, these experiments demonstrate the presence of rapidly turning-over proteins. The exportable proteins, the existence of which has been demonstrated in a preceding work [4], are a fraction of these proteins. Their origin seems to be also exclusively cytoplasmic. The site of elaboration of these exportable substances, their chemical nature, and their biological role, remain to be elucidated.

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It is a well-known fact that Bidder's organ is an undeveloped ovary. It is found in all adult male and in many adult female as well as in all juvenile male and female specimens of Bufo-species, and it is situated between the fatbody and the gonad. The fact that Bidder's organ is a potential ovary is not only derived from its morphology, but has also been proved experimentally by Harms (1923) and Ponse (1924), for after castration of male toads and after ovariectomy of female common toads (Bufo vulgaris Laur. (= B. bufo (L.)) Bidder's organ may develop into a normally functioning ovary. In 1931 Stohler compiled our knowledge concerning the distribution of Bidder's organ in the Bufonidae; in addition the occurrence of the organ was described in several other species in which hitherto it had not been recorded. We refer the reader interested in these species to Stohler's publication. After 1931 some useful papers appeared on the presence of Bidder's organ in certain Bufonids: Witschi (1933), Koch (1934), De Vos (1935) and Davis (1936). In these publications much older information was confirmed and some new facts were added (table I). In connection with investigations on the physiology and histophysiology of Bidder's organ in Bufo bufo (L.), carried out in our laboratory under the guidance of Prof. G. J. van Oordt, I have traced the presence of Bidder's organ in several species of Bufonids from the Amsterdam Zoological Museum and from the Leiden Rijksmuseum van Natuurlijke Historie, which had not yet been investigated by preceding authors in this respect 1). TABLE I

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