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During late foetal life and at birth the gonads of both Grey and Common seals are enlarged, and there are indications of stimulation in the rest of the reproductive tract. The enlargement of the gonads is due mainly to the presence of very large numbers of cells resembling interstitial cells. The presence of groups of small round cells resembling lymphocytes and of cells intermediate between them and fully developed interstitial cells suggests that the small round cells may be the precursors of the latter. Whether these cells are lymphocytes or primitive mesenchymal cells cannot be stated with certainty. The ovaries of foetal Common and Grey seals contain no follicles beyond the stage where oocytes are ensheathed in a single layer of epithelial cells, but more marked follicular development was present in the ovaries of a still‐born Californian sea lion. No evidence of spermatogenesis was seen in the testes of any foetal or neonatal male seal. Regression occurs rapidly after birth and is nearly complete at the age of two weeks. The interstitial cells disappear by a process similar to fatty degeneration. Retrogression is accompanied by a remarkable obliteration of many blood vessels supplying the outer part of the medulla and the cortex of the ovary. The stimulus responsible for the precocious enlargement of the foetal gonads is removed with the birth of the young. The rapid decrease of endocrine influence after birth suggests that the placenta may be implicated. In the adult Common and Grey seal the corpus luteum persists until the end of pregnancy and begins to degenerate a few days after parturition. Follicular development occurs in the maternal ovaries towards the end of pregnancy and in the immediate post‐parturient period. The maternal ovaries are probably a source of oestrogens, both before, during, and after parturition, and such oestrogens could cross the placenta and affect foetal tissues. The prostate of all male foetuses of both species towards the end of pregnancy and immediately after birth is markedly enlarged. There is proliferation and hypertrophy of the prostatic alveoli, dilation of the tubules, and an increase in the size of the epithelial cells. A week after birth the weight of the gland is reduced by more than one‐half. Persistence and hyperplasia of the uterus masculinus occurs in some foetal and newborn pups of the Common seal, but no similar enlargement was seen in the Grey seal. The thyroid in the late foetal and neonatal Common and Grey seals showed wide differences in their histological appearances. The thyroid glands from an adult lactating seal of each species were widely different. In the Grey seal it was almost completely atropic but in the Common seal it appeared to be intensely active. The amount of medulla present in the adrenal of foetal seals is small by comparison with that of the cortex. A large number of cortical islands packed with lipid is present in the medulla, often containing more lipid than the cortex itself. The capsule of the adrenal varies considerably in thickness ...

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