Bennet Mills Allen 137 



contain numerous globules of fat resulting from a process of fatty degen- 

 eration in these structures. Loisel, oo and 02, found the spherules of fat 

 to occur throughout the rudimentary sex gland of the 98-hour chick 

 embryo, and in the 5-day embryo of the California quail, and in sparrow 

 and guinea-pig embryos as well. According to him the primitive sex 

 cells lose them when they become spermatogonia, while certain germin- 

 ative and Sertollian cells contain fat globules up to the end of embryonic 

 life, when they disappear, to later reappear at the time of puberty and at 

 successive periods of sexual activity. There is thus a periodicity in their 

 formation at least in the sparrow, upon which form the greater part of 

 Loisel's work was done. He shows that the interstitial cells are filled 

 with fat globules at the same time that the cells of the seminiferous 

 tubules contain them, hence there is an interrelation between the two 

 sets of cells. Interstitial cells are rare if not non-existent in the testes 

 of adult birds, although they occur in great numbers in the testes of 

 adult mammals. 



He interprets the fat spherules described above as secretion products 

 and not as the products of degeneration. It seems to me unsafe to 

 hazard an opinion upon the physiological aspects of this process, yet it 

 certainly does result in the destruction, both immediate and remote, 

 of a large number of cells. It would hardly seem that in the present 

 state of our knowledge, Loisel is justified in his assertion that the 

 bright plumage assumed by birds during the breeding season is due to 

 any trophic stimulus imparted by this fatty substance. Coincident with 

 this process of fatty degeneration in these structures, certain cells of 

 the stroma suffer extensive modification, their cytoplasm becoming gran- 

 ular, acquiring a centrosome, centrosphere and definite cell limits. The 

 nuclei of these cells also enlarge and become spherical. These inter- 

 stitial cells are very numerous in the testis, but quite rare in the ovary. 

 In Iwth sex glands, they divide by mitosis. 



Plato, 97, Coert, 98, and Limon, 02, are unanimous in agreeing that 

 the interstitial cells arise from the stroma in both ovary and testis. 



3. Further Development of the Sex Glands. A. Testis. — The 

 peritoneum becomes less and less important in later stages, finally form- 

 ing a broken and almost vestigial covering of the sex gland. The albu- 

 ginea becomes thicker and more compact, but need be given little further 

 attention as its development is a very simple process. 



We have still to consider the development of the following elements : 



1. Seminiferous tubules. 



2. Eete tubules. 



3. Interstitial cells and stroma. 



