214 THE FOURTH DAY. [CHAP. 



The large size of the hinder part of the Wolffian body a* 

 compared with that of the anterior part is due to the presence of 

 the dorsally placed secondary tubules, whose development was 

 mentioned on p. 192. These are more numerous in the posterior 

 than in the anterior part of the Wolffian body. At the hind end 

 of the Wolffian body there are as many as four to each primary 

 tubule. 



The tubules, which from their contorted course are 

 in sections (Figs. 68, 71) seen cut at various angles, 

 possess an epithelium which is thicker than that of the 

 Wolffian duct. From this difference it is generally easy 

 to distinguish the sections of the tubules from those of 

 the duct. The glomeruli of the Malpighian bodies are 

 in sections of hardened embryos usually filled with 

 blood-corpuscles. 



Towards the hind end of the embryo, the projection 

 of the intermediate cell-mass spoken of above becomes 

 smaller and smaller, and the Wolffian duct is thus 

 brought nearer to the splanchnopleure, and in the 

 region of the hind-gut comes to lie close to the walls of 

 the alimentary canal. On the fourth day, the two ducts 

 meet and open into two horns, into which the side-walls 

 of the recently formed cloaca are at that time produced, 

 one on either side. 



As we shall afterwards see, the ducts of the perma- 

 nent kidneys and Miiller's duct also fall into these two 

 horns of the cloaca. 



The Wolffian bodies thus constituted perform the 

 offices of kidneys for the greater part of embryonic life. 

 In the chick they disappear before birth; but in most 

 of the Ichthyopsida they remain for life as the perma- 

 nent kidneys. 



Miillerian duct. After the establishment of the 



