4 BALFOUR AND SEDGWICK. 



this body is disconnected with the walls of the groove, and even 

 appears as if formed by a second invagination of the germinal 

 epithelium. In No. 3 this body becomes partially continuous 

 with the walls of the groove, and finally in No. 4 it becomes 

 completely continuous with the walls of the groove, and its lumen 

 communicates freely with the groove.^ 



The last trace of this body is seen on the upper wall of the 

 groove in No. 5. We beheve that the body (r,) represents the 

 ridge between the first and second grooves of the earlier stage ; 

 so that in passing from No. 3 to No. 5 we pass from the first to 

 the second groove. The meaning of the features of the body (r,) 

 in No. 2 we do not fully understand, but cannot regard them as 

 purely accidental, since we have met with more or less similar 

 features in other series of sections. The second groove becomes 

 gradually narrower, and finally is continued into the second ridge 

 (No. 8). The ridge contains a lumen, and is only connected 

 with the germinal epithehum by a narrow wall of cells. A 

 narrovr passage from the body cavity leads into that wall for a 

 short distance in No. 8, but it is probably merely the hinder end 

 of the groove of No. 7. The third groove appears in No. 11, 

 and opens into the lumen of the second ridge (^3) in No. 1^. In 

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

 duct (fg) 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 (^^3). In the next section this duct (^3) 

 is entirely separated from the germinal epithelium, and it may 

 be traced backwards through several sections until it terminates 

 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 

 epithelium {x) forming a second groove. It is especially con- 

 spicuous in the first six sections of the series. This fold some- 

 times 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 etudy 

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

 taken from a series slightly younger than b, shows this groove 

 {x) in its most exaggerated form. 



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

 ment 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 rudimentary grooves. 



' A deep focus of the rather thick section represented in No. 3 showed 

 the body much more nearly in the position it occupies in No. 4. 



