126 Embryonic Development of Ovary and Testis of Mammals 



In the rabbit, the conditions are essentially similar yet by no means 

 so clearly shown as in the pig. Difficulties in the study of these pro- 

 cesses in the rabbit are caused by the compactness of the tissues, the 

 smallness of the component cells, and the indistinctness of the limits 

 of the rete tubules. 



2. Sexual Differentiation. — It now remains to follow the ovary 

 and testis separately as they diverge in the process of further develop- 

 ment. Both are homologous, in that they have originated from an indif- 

 ferent rudiment in which a considerable complexity of structure has 

 become evident before it is possible to distinguish sexual differentiation. 



Unmistakable differences between ovary and testis can be discerned 

 in the 2.5 cm. embryo of the pig, less-marked differences being evident 

 in the embryo of 1.8 cm. lengih. A clear distinction between ovary and 

 testis is observable in the rabbit embryo of 14| days' age. 



Previous to the period of sex differentiation, the sex gland has taken 

 definite form, having become constricted off from the mesonephros, to 

 which it remains attached by the relatively narrow mesentery. A trans- 

 verse section shows it to be composed of the following tissues: (1) the 

 peritoneum, or germinal epithelium as it has been generally termed, 

 especially in the case of the ovary; (2) the albuginea, a term usually 

 applied to the subperitoneal connective tissue of the testis, but equally 

 applicable to the same zone in the ovary; (3) the sex cords; (4) the 

 interstitial stroma; (5) the distal ends of certain rete tubules that have 

 grown from the rete region into the anterior end of the sex gland. 



The prime features of sex differentiation are shown in the 2.5 cm. 

 pig embryo. The fundamental points are the further development of 

 the sex cords in the testis to form the seminiferous tubules and the 

 development of the peritoneum in the ovary to form the cords of Pfliiger. 

 These two sets of cords having a similar origin, but one which is suc- 

 cessive in point of time, are the structures in which the functional sex 

 products form. On the other hand, the sex cords of the ovary cease in 

 their growth and become the medullary cords — assuming the character 

 of the cords of Pfliiger. In the testis the peritoneum ceases to develop 

 and becomes flattened — finally almost disappearing in later stages. 



The albuginea layer is far thinner and more compact in the testis 

 than in the ovary. This is due to the fact that in the former it is much 

 more closely crowded against the peritoneum by the seminiferous tubules 

 than by the medullary cords in the case of the ovary. 



The peritoneum, cords of Pfliiger and medullary cords of the ovary, 

 together with the peritoneum and seminiferous tubules of the testis, 



