100 Emb];youic DeveloiDineut of Ovary and Testis of Mammals 



terior to this it is found to send numerous sliort projections into the 

 underlying stroma, and further back, these assume the character of 

 closely-crowded sex cords. At this place the rete tubules are few in 

 number and arise exclusively along a line very close to the mesentery. 

 They can be distinguished from^sex cords only by their isolation and 

 by their greater length. 



In general the rete tubules are made up largely of cells without 

 definite boundaries and in all other regards like those of the perito- 

 neum from which they originate. Only occasionally does one find a 

 primitive sex cell. 



1.8 cm. Embryo. — Sexual dift'erentiation is not yet clearly estab- 

 lished, although, in a vague way, the general distinctions mentioned 

 in connection with the 1.7 cm. stage are to be taken as criteria. 



The sex cords stand out in greater contrast to the stroma owing to 

 the fact that the cytoplasm of the component cells is much denser 

 than in the preceding stages (Plate IV, Fig. 13). In some places these 

 cords show a central lumen. This is not due to any regiilar process 

 of lumen formation, but has significance only in showing that there 

 is in each sex cord a line of weakness or rudimentary lumen which 

 owes its existence to the fact that these cords originated by a process 

 of invagination of the peritoneum. 



Exclusive of the primitive sex cells, there are two extreme types of 

 nuclei common to the sex cords, intercordal stroma and albuginea. 

 Those of the one kind are small and elongated, taking a deep diffuse 

 stain (Plate IV, Pig. 13). The nuclei of the other type are larger, 

 clearer and more rounded. There are all intermediate forms between 

 these two extremes. The larger nuclei predominate in the perito- 

 neum, the smaller variety characterize the albuginea, while the two 

 kinds appear in about equal number in the sex cords and in the 

 intercordal stroma. The marked similarity between the nuclei of 

 the sex cords and stroma is not surprising when one considers the fact 

 of their common origin. In the sex cords we shall term these the gerrain- 

 ative cells in contradistinction to the primitive sex cells. Transition 

 forms are found to unite the two distinct types of cells, thus showing 

 that certain of the germinative cells are being transformed into primi- 

 tive sex cells. The medium-sized germinative cells are probably the 

 most primitive; these form the sex cells on the one hand and on the 

 other the connective tissue cells. It is interesting to note that the 

 nuclei of many genninative cells are dividing by amitosis. 



The germinal epithelium of the sex gland of the 1.8 cm. embryo 

 does not contain any primitive sex cells clearly differentiated as such. 



