432 



COMPARATIVE ANATOMY 



proliferated towards the ectoderm. The nephrotome loses its connexion 

 with the epimere above, and together with the lateral outgrowth just 

 mentioned forms a pronephric tubule. In most vertebrate embryos, 

 pronephric tubules are primarily solid, but become hollow later. As they 

 grow laterally towards the ectoderm, the pronephric tubules also grow 

 posteriorly and unite to form a mass of rapidly dividing cells which con- 

 tinue to extend posteriorly as a cellular rod until they reach the cloaca. 

 Connexion with the cloaca is established and a lumen forms. In this 

 way is produced the primitive or pronephric duct. Although the proneph- 



MESONEPHRIC TUBULE 



MYOTOME 

 DERMATOME X^iik 



MESONEPHRIC TUBULES 

 NEURAL TUBE { 



<PHORDA 

 ««i-js -J /^ORTA 



! GLOMERULUS NEPHROSTOME | GEMITALRIDGE I PRIMITIVE DUCT 



PERITONEUM POSTCARDINAL VEIN MESENTERY COELOM 



Fig. 357. — Stereogram of the developing mesonephros at a stage later than that of 

 Fig- 355- (After Kingsley modified.) 



ric tubules which produced the primitive duct degenerate soon after their 

 appearance, the duct itself persists as the Wolffian or mesonephric duct, 

 so-called because it forms the outlet of the tubules of the mesonephros. 

 In some vertebrates, but apparently not in the human embryo, the 

 pronephric tubules open into the body cavity and persist, at least in part, 

 in the adult as the ostium tubae (the anterior opening of the oviduct). 



Mesonephros. The tubules of the mesonephros are formed from 

 nephrotomes posterior to those which form the pronephros. At its 

 full development in the human embryo, the mesonephros extends from 

 the sixth cervical to the third lumbar segment. Since as many as eighty- 



