622 EXISTENCE OF A HEAD-KIDNEY 



and opens into the lumen of the second ridge (r 2 ) in No. 12. In 

 No. 13 the groove is closed, and there is present in its place 

 a duct (r B ) connected with the germinal epithelium by a wall of 

 cells. This duct is the further development of the third ridge 

 of the last stage ; its lumen opens into the body-cavity through 

 the third and last groove (gr^). In the next section this duct 

 (r a ) is entirely separated from the germinal epithelium, and it 

 may be traced backwards through several sections until it term- 

 inates by a solid point, very much as in the last stage. 



In the figures of this series (B) there may be noticed on the 

 outer side of the Miillerian duct a fold of the germinal epithe- 

 lium (x) forming a second groove. It is especially conspicuous 

 in the first six sections of the series. This fold sometimes 

 becomes much deeper, and then forms a groove, the upper end 

 of which is close to the grooves of the head-kidney. It is very 

 often much deeper than these are, and without careful study 

 might easily be mistaken for one of these grooves. Fig. C, 

 taken from a series slightly younger than B, shews this groove 

 (x) in its most exaggerated form. 



The stage we have just described is that of the fullest de- 

 velopment of the head-kidney. In it, as in all the previous 

 stages, there appear to be only three main openings into the 

 body-cavity ; but we have met in some of our sections with 

 indications of the possible presence of one or two extra rudi- 

 mentary grooves. 



In an embryo not very much older than the one last de- 

 scribed the atrophy of the head-kidney is nearly completed, 

 and there is present but a single groove opening into the body- 

 cavity. 



In series D (PL 28) are represented a number of sections 

 from an embryo at this stage. Nos. I and 2 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 head -kidney) 



