604 MUSCLES AND SKELETON CHAP. 



in connection with the functional renal tubules, the 

 Wolffian duct serves as a spermiduct mainly if not 

 entirely (dogfish, p. 4^6; mammal, p. 538). 



In most Vertebrates the oviducts arise quite inde- 

 pendently of the mesonephric ducts, each being formed 

 as a groove of the ccelomic epithelium which becomes 

 closed in and grows from before backwards to the 

 cloaca, its anterior opening into the ccelome representing 

 a nephrostome of the pronephros. 



In the higher Vertebrates, in which the kidney of the 

 adult is a metanephros (p. 537), the only parts of the 

 mesonephros and its duct which persist as more than a 

 vestige are the epididymis and spermiduct of the male. 



The gonads arise as ridges covered by ccelomic 

 epithelium on the dorsal wall of the body-cavity close 

 to the inner side of the developing kidneys. Their 

 epithelium is known as germinal epithelium, and from it 

 either ova or sperms are eventually developed (pp. 194- 

 196, and 566). 



The majority of the muscles are developed, as we 

 have seen (pp. 203 and 588), from the mesodermal 

 segments, others arising from the parietal and visceral 

 layers of the mesoderm. 



The first part of the endoskeleton to be formed is the 

 notochord (pp. 203 and 580), developed primarily from 

 the endoderm, but in the chick and rabbit arising from 

 the central part of the mass of cells in the primitive 

 streak (Fig. 153, G), in which the layers become in- 

 distinguishable from one another (p. 583). In the meso- 

 derm surrounding the notochord cartilages appear and 

 give rise to the vertebra, the notochord becoming con- 

 stricted by the ingrowing cartilage and eventually dis- 

 appearing more or less completely (p. 441) : it at first 



