EXISTENCE OF HEAD-KIDNEY IN THE EMBRYO CHICK. 5 



In an embryo not very much older than the one last described 

 the atrophy of the head-kiduey is nearly completed^ and there is 

 present but a single groove opening into the body cavity. 



In series d (PI. II) are represented a number of sections 

 from an embryo at this stage. Nos. 1 and £ are sections through 

 the hind end of the single groove now present. Its walls are 

 widely separated from the Wolffian duct in front, but approach 

 close to it at the hinder termination of the groove (No. 2). The 

 features of the single groove present at this stage agree closely 

 with those of the anterior groove of the previous stages. The 

 groove is continued into a duct — the Miillerian duct (as it may 

 now be called, but in a previous stage the hollow ridge connecting 

 the first and second grooves of the liead-kidney) — which, after 

 becoming nearly separated from the germinal epithelium, is again 

 connected to it by a mass of cells at two points (Nos. 5, 6, and 

 8). The germinal epithelium is slightly grooved and is much 

 reduced in thickness at these points of contact ( ^;'2 and gr-^), and 

 we believe that they are the remnants of the posterior grooves of 

 the head-kidney present at an earlier stage. 



The Miillerian duct has by this stage grown much further 

 backwards, but the peculiarities of this part of it are treated in a 

 subsequent section. 



We consider that, taking into account the rudiments we have 

 just described, as well as the fact that the features of the single 

 groove at this stage correspond with those of the anterior groove 

 at an earlier stage, we are fully justified in concluding that the 

 permanent ahdominal opening^f the Mullerian duct corresponds 

 with the anterior of our three grooves. 



Although we have, on account of their indefiniteuess, avoided 

 giving the ages of the chicks in which the successive changes of 

 the head-kidney may be observed, we may, perhaps, state that all 

 the changes we have described are usually completed between 

 the 90th and 120th hour of incubation. 



The Glomerulus of the Head-Kidney. 



In connection with the head-kidney in Amphibians there is 

 present, as is well known, a peculiar vascular body usually de- 

 scribed as the glomerulus of the head-kidney. We have found 

 in the chick a body so completely answering to this glomerulus 

 that we have hardly any hesitation in identifying it as such. 



In the chick the glomerulus is paired, and consists of a vascular 

 outgrowth or ridge projecting into the body cavity on each side 

 at the root of the mesentery. It extends from the anterior end 

 of the Wolffian body to the point where the foremost opening 

 of the head-kidney commences. We have found it at a period 

 slightly earlier than that of the first development of the head- 



