88 



Basic Structure of Vertebrates 



Amphibians give some evidence that the anterior part of the Miil- 

 lerian duct is derived from pronephric structures, as seems to be true 

 for sharks. In reptiles, birds, and mammals, the Mullerian duct de- 

 velops independently of the Wolffian duct 

 but very closely parallel to it (Fig. 87), and 

 there is some evidence that the anterior 

 region and ostium of the Mullerian duct 

 arise in very intimate relation to the pro- 

 nephros. On the whole, it seems somewhat 

 likely that the oviducts are modified deriva- 

 tives of pronephric structures. 



Differentiation of Sex 



The vertebrate embryo, during its earlier 

 stages of development, is anatomically in- 

 different as to sex. Every embryo contains 

 the rudiments of all of the urinogenital or- 

 gans of both sexes. As the gonads, in later 

 stages, acquire their definitive differentiation 

 as ovaries or testes, the embryonic parts 

 appropriate to the sex indicated by the 

 gonad likewise proceed to acquire complete 

 and functional differentiation. But the em- 

 bryonic rudiments corresponding to the other 

 sex either cease developing and gradually 

 dwindle away or, at most, progress only to 

 the point of becoming relatively small and 

 imperfectly developed parts having little 



Fig. 87. Transverse sec- 

 tion through the urinogeni- 

 tal region of a four-day 

 chick embryo, (g) Mesoder- 

 mal epithelium (perito- 

 neum ) of genital ridge; (m) 

 infolding of peritoneum to 

 form Miillerian duct; (ms) 

 mesentery; (s) mesen- 

 chyme cells which give rise 

 to the stroma (nongenital 

 tissue) of gonad; {t) meso- 

 nephric tubules; (w) Wolf- 

 fian duct. (After Waldeyer. 

 Courtesy, Kingsley : " Com- 

 parative Anatomy of Verte- 

 brates," Philadelphia, The 

 Blakiston Company.) 



or no functional significance. Adult males 

 usually possess vestiges of the Mullerian ducts, and adult females hav- 

 ing the metanephros may possess vestiges of mesonephric structures. 



