3i6 



CHORDATE ANATOMY 



segments seven to fourteen. The first of the pronephric tubules makes its 

 appearance in a 1.7 mm. embryo, and all eight tubules are formed by the 

 time the embryo has reached a length of 2.5 mm. Degeneration begins 

 soon and the anterior tubules disappear before the posterior ones are 

 differentiated. 



The development of pronephric tubules is initiated by the parietal 

 layer of the nephrotome or intermediate cell mass from which cells are 

 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 



SPINAL CORD. 



SPINAL 

 GANGLION-/. 



MESONEPHRIC TUBULES 

 PRIMITIVE DUCT 



GLOMERULUS 

 POSTCARDINAL VEIN 



MESENTERY 



PRONEPHRIC TUBULES 



Fig. 280. — Stereogram of the developing pronephros and inesonephros. (After Kingsley, 



modified.) 



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- 

 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, at least in part, persist 

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



