1426 



THE URINOGENITAL ORGANS 



other hand, maintains that in the skate they are not derived from this epithelium, but are probably 

 formed during the later stages of cell cleavage, before there is any trace of an embryo; and a 

 similar view was advanced by Nussbaum as to their origin in amphibia. Beard says: "At the 

 close of segmentation many of the future germ cells lie in the segmentation cavity just beneath 

 the site of the future embryo, and there is no doubt they subsequently wander into it." The 

 germ cells, "after they enter the resting phase, are sharply marked off from the cells of the em- 

 bryo by entire absence of mitoses among them." They can be further recognized by their 

 irregular form and amoeboid processes, and by the fact that their cytoplasm has no affinity for 

 ordinary stains, but assumes a brownish tinge when treated by osmic acid. The path along 

 which they travel into the embryo is a very definite one viz., "from the yolk sac upward between 

 the splanchopleure and gut in the hinder portion of the embryo." This pathway, named by 

 Beard the germinal path, "leads them directly to the position which they ought finally to take 

 up in the 'germinal ridge.' " A considerable number apparently never reach their proper desti- 

 nation, since "vagrant germ cells are found in all sorts of places, but more particularly on the 

 mesentery." Some of these may possibly find their way into the germinal ridge; some probably 

 undergo atrophy, while others may persist and become the seat of dermoid tumors. 



Ovarian tube of epithelium. 



Blood-vessel.- 

 Graafian follicle. 



Germinal epithelium. 



^ 

 Primitive ova. 



Cell nest. 



FIG. 1185. Section of the ovary of a newly born child. (Waldeyer.) 



The testis is developed in a very similar way to the ovary. Like the ovary, in its earliest 

 stages it consists of a central mass of connective tissue covered by germinal epithelium, among 

 which larger cells, the primitive sperm cells, are seen. These are carried into the subjacent stroma 

 by tubes of germinal epithelium, which form the lining of the seminiferous tubules, while the 

 primitive sperm cells form the spermatogonia. The seminiferous tubules become connected 

 with outgrowths from the Wolffian body, which, as before mentioned, form the rete testis and 

 vasa efferentia. 



Descent of the Testes. The testes, at an early period of fetal life, are placed at the back part 

 of the abdominal cavity, behind the peritoneum and a little below the kidneys; their anterior 

 surfaces and sides are invested by peritoneum. About the third month of intrauterine life a 

 peculiar structure, the gubernaculum testis, makes its appearance. This is at first a slender 

 band, extending from that part of the skin of the groin which afterward forms the scrotum 

 through the inguinal canal to the body and epididymis of the testis, and is then continued upward 

 in front of the kidney toward the Diaphragm. As development advances, the peritoneum cover- 

 ing the testis encloses it and forms a mesentery, the mesorchium, which encloses also the guber- 

 naculum and forms two folds, one above the testis and the other below it. The one above the 

 testis is the plica vascularis, and contains ultimately the spermatic vessels; the one below, the 

 plica gubernatrix, contains the lower part of the gubernaculum, which has now grown into a 

 thick cord; it terminates below at the internal ring in a tube of peritoneum, the processus vagi- 

 nalis, which protrudes itself down the inguinal canal. The lower part of the gubernaculum by 

 the fifth month has become a thick cord, while the upper part has disappeared. The lower part 

 can now be seen to consist of a central core of unstriped muscle fibre, and outside this of a firm 

 layer of striped elements, connected, behind the peritoneum, with the abdominal wall. As the 

 scrotum develops, the main portion of the lower end of the gubernaculum is carried with the 

 skin to which it is attached to the bottom of this pouch ; other bands are carried to the inner side 

 of the thigh and to the perineum. The fold of peritoneum, constituting the processus vaginalis, 

 projects itself downward into the inguinal canal, and emerges at the external abdominal ring, 



