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This research was undertaken to examine the way that social networks are formed and change and the factors which influence these changes. The conjugal pair was taken as the focus of the network and married couples selected from a district of Edinburgh to form a research set. The project was carried out by means of a series of interviews with the members of the research set and participant observation. The data suggested the following network characteristics. It was found that the main factor affecting network changes was the stage in the developmental cycle of the family. In the different stages of the developmental cycle, network recruitment was from different sectors. Thus in the first stage of the development cycle most network members were old childhood friends, in the second stage recruitment was mostly from the neighbourhood, in the third stage network members were recruited from voluntary associations and the work situation and in the final stage the network was made up mostly of kin. The results were similar for both the working class and the middle class members of the research set. Social class was found to influence the way social contacts were made within the sectors, thus the way in which network links were formed differed between the middle class couples and the working class couples. Another finding was that the networks of the research set were of low density, but that the density of the individual sectors was high. This suggests that urban married couples have low density social networks but that high density areas may be found in the sectors of the network. Since the project showed that the stage in the developmental cycle of the family was largely responsible for the sectors from which recruitment to the network took place, it can be suggested that the high density areas of a social network are dependent on the stage in the family cycle of the conjugal pair. High density networks are likely to produce norm enforcing groups and therefore it seems that the stage in the developmental cycle is an important factor in influencing network changes and characteristics and also in determining in which sectors norm enforcing groups will exist.

1. Autogamy, or self-fertilisation in the marine ciliate Euplotes minuta has been studied by genetic and cytological means. The naturally occurring, between stock variation in four characters was used to contrast the genetic consequences of both conjugation and autogamy. 2. The existence of a number of different mating types allows crosses to be made, and the inheritance of the various characters to be analysed. All mating type alleles behave in crosses as if they are alternatives at a single locus, and as if there is a serial system of dominance - whereby an allele assigned a number high in the series is dorninant over all alleles lower in the series. Stock A25, previously described as being heterozygous at the mating type locus (Nobili ana Luporini, 1967) , was found to have the genotype mt⁷/mt⁷. 3. In this species, two types of stock are available; one type possesses the ability to pass through autogamy, the other type does not. The F₁ clones derived from crossing autogamous and non-autogamous parents were all capable of autogamy. The fact that the ratio of autogamous to non-autogamous clones was not significantly different from, firstly a 1 : 1 ratio in backcross to one non-autogamous parent, and secondly a 3 : 1 ratio in an F₂ derived by crossing two F₁ clones, means that this trait could be controlled by a single locus, at which an allele which permits autogamy is dominant to an allele which does not allow this process to occur. Non-autogamous stocks must be homozygous for the non-autogamous alleles, and autogamous stock A25 must be homozygous for the allele which confers autogamy. 4. Antisera were prepared against two strains of E.minuta. when prepared against an autogamous strain, the antiserum is efficient at immobilising the homologous and heterologous autogamous stocks, but gives a weak reaction against the non-autogamous stocks. An antiserum against a non-autogamous strain is also capable of discriminating between the two classes of stocks. Serotype was not inherited according to simple Mendelian rules. It is suggested that several loci may interact in the determination of this character. 5. By the use of electrophoresis of cell extracts, followed by detection of localised enzyme activity on a starch-gel, stocks were found to differ with respect to an esterase termed Es-l. This enzyme was characterised by inhibitor studies. Whereas the non-autogamous stocks used for genetic purposes possess an active form of Es-l, and are said to carry the allele Es-lᵇ, the autogamous stock A25 has no Es-l activity and carries only the silent Es-lᵃ alleles. The presence of esterase Es-l is controlled by a single gene; the allele Es-lᵇ being dominant to the silent allele Es-lᵃ. The results are consistent with the enzyme being a single polypeptide chain. 6. There was no genetic linkage between any of the four characters studied. 7. F₁ clones heterozygous at the loci controlling the four characters were passed through autogamy. Although after conjugation the segregation ratios for those traits which are certainly controlled by single genes, never deviated significantly from expectation, the phenotype of the F₁ was in most cases retained in the F₂ from autogamy. At the mating type and Es-l loci the recessive homozygote appeared in only 1 in 30 exautogamous clones. 8. Nuclear events at conjugation and autogamy were studied by Feulgen and Giemsa staining. The micronucleus first divides mitotically and the two products undergo meiosis. Two of the 8 haploid products of the second meiotic division persist and undergo a further division. Two of the four products are the gametic nuclei. After synkaryon formation there is normally one post-zygotic division; one product becomes the new micronucleus, the other enlarges into the macronuclear anlage. In the exconjugants this anlage differentiates to produce the sickle-shaped adult macronucleus. After autogamy the macronucleus rarely develops beyond the anlage stage; the anlage disintegrates and most animals which survive autogamy do so by regeneration of one or two fragments of the old macronucleus which are still present in the cell. 9. Knowledge of the sequence of nuclear events preceding conjugation and autogamy makes it possible to compute the expectations for the various genotypes after fertilisation. In P.aurelia, only 1 haploid nucleus survives beyond the second meiotic division, and produces gametes; therefore, exconjugants are alike, and exautogamous animals are homozygous. Due to the persistence, in E.minuta, of 2 haploid nuclei which appear to be the products of different meioses, gametic nuclei may be sisters, or non-sisters. Cytological data and genotypic ratios support the conclusion that in a cross Aa x aa the two exconjugants shculd be alike in 2/3 of the pairs, and unlike in 1/3. Crosses with E.vannus, E.crassus (Heckmann, 1963, 1964) and E.minuta fit this expectation. The expected ratio for synkarya after autogamy of animals of genotype Aa, is lAA : lAa : laa. The genetic evidence shows that this expectation is clearly not fulfilled. 10. Macronuclear regeneration produces heterocaryons in other ciliates. The possibility that exautogamous E.minuta are heterocaryons is discussed. It is concluded that macronuclear regeneration is not an artefact of laboratory culture, but has an advantage over genuine autogamy in that maximum heterozygosity is retained. Autogamy starts a new phase of the life cycle in E.minuta; animals initially being immature. The, fact that autogamy can counteract the effects of senescence enables the nature of the rejuvenation process and theories of ageing to be reconsidered.