462 STRUCTURE OF VERTEBRATA. 



The segmental or pronephric duct on each side is, at 

 any rate in some of the lower Vertebrates, divided into two 

 ducts, the Miillerian duct and the mesonephric or Wolffian 

 duct. In the Amniota the origin of the Miillerian duct 

 is still somewhat obscure. It becomes the genital duct or 

 oviduct of the female, while in the male the Wolffian duct 

 becomes the genital duct or vas deferens. 



The ureters or ducts from the persistent functional 

 kidneys are either the original archinephric or segmental 

 ducts (e.g., in Cyclostomata), or the Wolffian ducts (in 

 Amphibians), or special posterior derivations of the latter. 



Suprarenal Bodies. These are found in most Vertebrates near the 

 reproductive organs and kidneys. Structurally, each shows a distinction 

 into a cortical and a medullary zone. It is usually asserted that these 

 two areas have a different origin, the medullary region being derived 

 from the sympathetic nervous system, the cortex from the pronephros. 

 On the other hand, some investigators consider that the medulla is 

 derived from metamorphosed cortical cells. Nor is the origin of the 

 cortical part beyond dispute, for by some it is said to originate from a 

 degeneration of the most anterior portion of the germinal epithelium, or 

 from this in association with a part of the primitive kidney. 



With regard to function, there is even more uncertainty. The supra- 

 renal bodies are relatively very large in embryonic life, but fail to 

 maintain their primitively rapid rate of growth. It has been suggested 

 that they assist in breaking down or disposing of waste pigment. 



Reproductive System. 



The ovaries and testes are developed from a ridge formed 

 by a part of the epithelium lining the abdominal cavity, 

 this ridge constituting the so-called germinal epithelium. 



In the male, the proliferating germinal epithelium is 

 divided by embryonic connective tissue into numerous 

 follicles. The cells of the follicles form seminal mother 

 cells, which, by their ultimate divisions, give rise to sper- 

 matozoa. From the mesonephros, tubules grow out to the 

 embryonic testes ; these form the collecting tubes of the 

 organ and open into the mesonephric duct, the vas deferens 

 of the adult. This is the most typical Vertebrate condition, 

 but is not universal. (See Table, page 461.) 



In the female, the ovary is similarly divided up into 

 follicles. In this case, however, differentiation sets in 

 among the originally equivalent cells of the follicle. One 

 cell in each follicle is more successful than its neighbours, 



