IN THE EMBRYO CHICK. 623 



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 (gr z and gr a \ 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 abdominal opening of the Miillerian duct corresponds 

 with the anterior of our three grooves. 



Although we have, on account of their indefiniteness, 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 Qoth and i2oth 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 vas- 

 cular 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 open- 

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

 period slightly earlier than that of the first development of the 

 head-kidney. It is represented in figs. E and F, PI. 28 gl, and is 

 seen to form a somewhat irregular projection into the body- 

 cavity, covered by a continuation of the peritoneal epithelium, 



