202 OUTLINES OF CHORD ATE DEVELOPMENT 



ceding chapter (Figs. 24, 25). In the male each of the paired 

 testes, suspended from the dorsal body wall by a double fold of 

 peritoneum, the mesorchium, is connected by a series of small 

 tubules, the vasa efferentia, with the upper end of the Wolffian 

 duct or mesonephric duct. The vasa efferentia are modified 

 mesonephric tubules, and the mesonephric duct therefore 

 functions here both as excretory (ureter) and reproductive 

 (vas deferens) duct, i.e., as a urinogenital duct. The homolog 

 of the oviduct or MuUerian duct of the female, is represented 

 in the male by a vestigial cord. 



In the female the ovaries are similarly suspended by mes- 

 ovaria. They are not directly connected with the reproductive 

 ducts or oviducts, and consequently, in this sex the Wolffian 

 or mesonephric duct remains purely excretory in function 

 (ureter). The oviduct is a long convoluted tube which de- 

 velops from the peritoneal epithelium, quite independently of 

 the excretory ducts. 



A. THE GONODUCTS 



It will be convenient to describe the formation of the gono- 

 ducts first. The vas deferens of the male is the original meso- 

 nephric duct or ureter, and nothing need be added to the account 

 of its development given above, save to point out that in con- 

 nection with each duct a glandular seminal vesicle develops, 

 just in front of the opening of the duct into the cloaca. In the 

 female the mesonephric duct remains unmodified, and the 

 seminal vesicles are barely represented. 



The Miillerian ducts for a long time develop similarly in both 

 sexes, although they become fully developed and functional, as 

 the oviducts, only in the female. The Miillerian duct appears 

 during the early stages of the degeneration of the pronephros, 

 just beneath the common nephrostome, the formation of which 

 has been mentioned. Here a peritoneal proliferation forms an 

 elongated thickening along the dorso-lateral wall of the body 

 cavity. From the dorsal border of this thickening a thin shelf 

 of cells forms and projects downward (Fig. 75), parallel with 

 the peritoneal thickening. The free lower margin of this shelf 



