IN THE EMBRYO CHICK. 627 



posterior part also. It then forms a duct, ending in front by a 

 groove in free communication with the body-cavity, and term- 

 inating blindly behind. The open groove in front at first 

 deepens, and then becomes partially constricted into a duct, 

 which elongates and becomes convoluted, but remains in com- 

 munication with the body-cavity by from two to four (according 

 to the species) separate openings. The manner in which the 

 primitive single opening is related to the secondary openings is 

 not fully understood. By these changes there is formed out of 

 the primitive groove an anterior glandular body, communicating 

 with the body-cavity by several apertures, and a posterior duct, 

 which carries off the secretion of the gland, and which, though 

 blind at first, eventually opens into the cloaca. In addition to 

 these parts there is also formed on each side of the mesentery, 

 opposite the peritoneal openings, a very vascular projection into 

 this part of the body-cavity, which is known as the glomerulus of 

 the head-kidney, and which very closely resembles in structure 

 and position the body to which we have assigned the same name 

 in the chick. 



The primitive segmental duct is at first only the duct for 

 the head-kidney, but on the formation of the posterior parts of 

 the kidney (Wolffian body, &c.) it becomes the duct for these 

 also. 



After the Wolffian bodies have attained to a considerable 

 development, the head-kidney undergoes atrophy, and its peri- 

 toneal openings become successively closed from before back- 

 wards. At this period the formation of the Miillerian duct takes 

 place. It is a solid constriction of the ventral or lateral wall of 

 the segmental duct, which subsequently becomes hollow, and 

 acquires an opening into the body-cavity quite independent of tJie 

 openings of tlie head-kidney. 



The similarity in development and structure between the 

 head-kidney in Amphibia and the body we have identified as 

 such in Aves, is to our minds too striking to be denied. Both 

 consist of two parts (i) a somewhat convoluted longitudinal 

 canal, with a certain number of peritoneal openings; (2) a vascu- 

 lar prominence at the root of the mesentery, which forms a 

 glomerulus. As to the identity in position of the two organs we 

 hope to deal with that more fully in speaking of the general 



