Development of the Fourth Day 233 



throughout Hfe as the animal's "backbone" or 

 primitive vertebral column. 



The Wolffian bodies. — ^By the end of this 

 day, the tubules of the anterior end of the 

 Wolffian body have disappeared ; but the 

 tubules of the posterior end have increased 

 in size, and become very much convoluted, so 

 that the intermediate cell-mass, in which they 

 lie, projects still more prominently into the 

 body-cavity. In cross-sections of the chick, 

 the convoluted Wolffian tubules are seen cut 

 across at various angles : they may usually be 

 distinguished from the Wolffian duct by the 

 fact that their walls are made up of a some- 

 what thicker epithelium than that of the duct 

 (Figs. 73 and 74). The glomeruli of the 

 Malpighian bodies are usually seen, in cross- 

 sections, to be filled with blood corpuscles. 



As has been said, the Wolffian body, or 

 mesonephros, functions as the kidney during 

 the greater part of the embryonic life of the 

 chick, but disappears before hatching, or at 

 least ceases to function, and is replaced by the 

 permanent kidneys. In most of the fishes and 

 amphibians the Wolffian body is the func- 

 tional kidney throughout life. 



The pronephros and the Miillerian duct. — " Towards 



