318 THE CHICK. 



merulus, then arises from the side wall of each tubule, and pro- 

 jects into its cavity ; by further enlargement, the glomeruli 

 grow out through the expanded mouths of the tubules, or nephro- 

 stomes, so as to hang freely into the body cavity. These changes 

 are probably of a degenerative character, for shortly afterwards 

 both the glomeruli and the tubules disappear completely. 

 Further back in the body, but still in front of the sixteenth 

 somite, the WolflSan tubules develop differently ; the nephro- 

 stomes close, the tubules separate from the peritoneum, and 

 then become dilated to form Malpighian bodies, into which little 

 vascular tufts or glomeruli, derived from the aorta, soon pene- 

 trate. 



In the posterior part of the WolflBan body, from about the 

 sixteenth to the thirtieth somites, there are no nephrostomes. 

 The Wolffian tubules in this region have no connection with the 

 peritoneal epithelium, but arise from the first in the mesoblast, 

 appearing as oval vesicles, which by elongation become the 

 Wolffian tubules. These acquire openings into the Wolffian duct, 

 and dilate at their opposite ends to form Malpighian bodies. 

 After the first-formed tubules are completed, others arise in the 

 same manner, and usually nearer the dorsal surface. Partly 

 owing to this increase in the number of the Wolffian tubules, 

 and partly owing to each tubule increasing greatly in length 

 and becoming much convoluted, the Wolffian body soon attains 

 a considerable size, causing a marked ridge-like projection of 

 the intermediate ceU mass into the body cavity, along each side 

 of the mid-dorsal line. 



4. The Head Kidney and the Mlillerian Duct. 



Towards the end of the fourth day, three pit-like involutions 

 of the peritoneal epithelium appear, one behind another, close to 

 the outer side of the Wolffian duct, and three or four somites 

 behind its anterior end. A ridge-like thickening of the peritoneal 

 epithelium connects the three pits of each side with one another, 

 and grows backwards behind the third pit as a solid rod of cells, 

 lying along the outer side of the Wolffian duct, and very close 

 to this. 



This rod soon becomes tubular, ending blindly behind, but 

 opening in front into the body cavity through the three pits. 

 These three pits form the head-kidney of the chick embryo, and 



