Bennet Mills Allen 135 



does not jDroperly come within the limits of this work, yet I cannot re- 

 frain from pointing out the very close resemblance betw^een the inter- 

 stitial and lutein cells (Plate VII, Figs. 29 and 30). It seems quite 

 improbable that two groups of cells, almost identical as these are, could 

 have arisen from such diverse elements as the connective tissue cells of 

 the theca interna, on the one hand, and the granulosa cells on the other. 



Successive generations of lutein and interstitial cells push the earlier- 

 formed groups of cells toward the center of the ovary, where they 

 undergo hyalin degeneration. De Sinety, 77, finds in the human 

 subject that the number of atretic follicles is greater during pregnancy 

 than at other times. I cannot substantiate this; but am inclined to con- 

 sider pregnancy to make no difference in their number in the rabbit. 

 This view I can support by a number of ovaries, taken from immature 

 rabbits, mature virgins, immature virgins, pregnant animals and one 

 taken from a rabbit that had borne young but had been isolated for 

 three months. 



4. Primitive Sex Cells. — The primitive sex cells occur from the 

 earliest stages studied (pig, 6 mm. length; rabbit, 13-day embryo) on 

 through all later stages of development. Similar cells have been found 

 in the earliest stages of the Elasmobranchs by Beard, 00, 02, 03, Eabl, 

 98, Woods, 02, and in the Teleost by Eigenmann, 91. I have myself 

 found the large yolk-filled primitive sex cells of these authors in turtle 

 embryos (Trionyx) and may say that I am now at work upon this 

 subject. It is too early to give results, but it may be stated with certainty 

 that these cells occur in the embryo of 3 mm. length and are seen to 

 be apparently migrating from the entoderm through the splanchnopleuric 

 mesoderm to the point where the latter joins the somatopleuric mesoderm. 

 It is here that the sex glands are to form. This observation corresponds 

 with that of Beard, 03. 



The nuclei of such cells are larger than those of the entoderm and 

 mesoderm among which they lie, but resemble them at this stage in the 

 fact that they show very little chromatin material as contrasted with 

 the very pronounced chromatin network which they show in later stages 

 (1.5 cm. embryo). In this stage the large yolk spherules are found to 

 be breaking up into small granules which remain in one or two clumps 

 in the cytoplasm. These cells at this stage show a very marked resem- 

 blance to the primitive sex cells of the pig and rabbit. In fact I have 

 no hesitation in identifying them as their homologues. In the pig and 

 rabbit they are found not only in the genital ridge, but outside of it as 

 well, in the earlier stages. So far as my own work goes, I have found 

 them in the mesentery of the alimentary canal. Eigenmann, 91, found 



