620 EXISTENCE OF A HEAD-KIDNEY 



extremity than in front. It is replaced by groove number two, 

 which appears as if formed by the reverse process to that by 

 which the ridge arose, viz., by a hollowing out of the ridge on 

 the side towards the body-cavity. The wall of the second 

 groove is, after a few sections, continued into a second ridge or 

 thickening of the germinal epithelium, which, however, is so 

 faintly marked as to be hardly visible in its middle part. In its 

 turn this ridge is replaced by the third and last groove. This 

 vanishes after one or two sections, and behind the point of its 

 disappearance we have failed to find any further traces of the 

 head-kidney. The whole formation extends through about 

 twenty-four of our sections and one and a half segments (muscle- 

 plates). 



We have represented (Plate 27, Series A, Nos. i 10) a fairly 

 complete series of sections through part of the head-kidney of 

 an embryo slightly older than that last described, containing 

 the second and third grooves and accessory parts. The connec- 

 tion between the grooves and the ridges is very well illustrated 

 in Nos. 3, 4, and 5 of this series. In No. 3 we have a pro- 

 minent ridge, in the interior of which there appears in No. 4 

 a groove, which becomes gradually wider in Nos. 5 and 6. 

 Both the grooves and ridges are better marked in this than in 

 the younger stage ; but the chief difference between the two 

 stages consists in the third groove no longer forming the hinder- 

 most limit of the head-kidney. Instead of this, the last groove 

 (No. 7) terminates by the upper part of its walls becoming con- 

 stricted off as a separate rod, which appears at first to contain 

 a lumen continuous with the open groove. This rod (Nos. 7, 8, 

 9, 10) situated between the germinal epithelium and Wolffian 

 duct is continued backward for some sections. It finally termi- 

 nates by a pointed extremity, composed of not more than two 

 cells abreast (Nos. 8 10). 



Our third stage, sections of which are represented in series B 

 (Plate 27), is considerably advanced beyond that last described. 

 The most important change which has been effected concerns 

 the ridges connecting the successive grooves. A lumen has 

 appeared in each of these, which seems to open at both ends 

 into the adjacent grooves. At the same time the cells, which 

 previously constituted the ridge, have become (except where 



