IN THE EMBRYO CHICK. 625 



does not apparently alter in constitution, and we have not 

 thought it worth while giving any further representations of it 

 during the later stages of its existence. 



Summary of the development of tJie head-kidney and glome- 

 rulus. The first rudiment of the head-kidney arises as three 

 successive grooves in the thickened germinal epithelium, con- 

 nected by ridges, and situated some way behind the front end 

 of the Wolffian duct. In the next stage the three ridges con- 

 necting the grooves have become more marked, and in each of 

 them a lumen has appeared, opening at both extremities into 

 the adjoining grooves. Still later the ridges become more or 

 less completely detached from the peritoneal epithelium, and 

 the whole head-kidney then consists of a slightly convoluted 

 duct, with, at the least, three peritoneal openings, which is pos- 

 teriorly continued into the Mullerian duct. Still later the head- 

 kidney atrophies, its two posterior openings vanishing, and its 

 anterior opening remaining as the permanent opening of the 

 Mullerian duct. The glomerulus arises as a vascular prominence 

 at the root of the mesentery, slightly prior in point of time to 

 the head-kidney, and slightly more forward than it in position. 

 We have not traced its atrophy. 



We stated in our preliminary paper that the peculiar struc- 

 tures we had interpreted as the head-kidney had completely 

 escaped the attention of previous observers, though we called 

 attention to a well-known figure of Waldeyer's (copied in the 

 Elements of Embryology, fig. 51). In this figure a connection 

 between the germinal epithelium and the Mullerian duct is 

 drawn, which is probably part of the head-kidney, and may be 

 compared with our figures (Series B, No. 8, and Series D, No. 4). 

 Since we made the above statement, Dr Gasser has called .our 

 attention to a passage in his valuable memoir on " The Develop- 

 ment of the Allantois 1 ," in which certain structures are described 

 which are, perhaps, identical with our head-kidney. The fol- 

 lowing is a translation of the passage : 



"In the upper region of M tiller's duct I have often observed 

 small canals, especially in the later stages of development, which 

 appear as a kind of doubling of the duct, and run for a short 



1 Beitrdge zur Entwickclnngsgeschichte d. Allantois dcr Mitllcr'schen Gange H. dcs 

 Afters. Frankfurt, 1874. 



