EXISTENCE OF HEAD-KIDNEY IN THE EMBRYO CHICK. 6 



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 I, series a, Nos. 1 — 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 J 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 diflFerence between the two 

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

 dermost limit of the head-kidney. Instead of this, the last 

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

 constricted 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 I), 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 they 

 are continuous with the walls of the grooves) partially con- 

 stricted off from the germinal epithelium. The ridges, in fact, 

 now form ducts situated in the stroma of the ovarian ridge, in 

 the space between the Wolffian duct and the germinal epithe- 

 lium. The duct continuous with the last groove is somewhat 

 longer than before. In a general way, the head-kidney may now 

 be described as a duct opening into the body cavity by three 

 groove-like apertures, and continuous behind with the rudiment 

 of the true Miillerian duct. Although the general constitution 

 of the head-kidney at this stage is fairly simple, there are a few 

 features in our sections which we do not fully understand, 

 and a few points about the organ which deserve a rather fuller 

 description than we have given in this general sketch. 



The anterior groove (No. 1 — 3, series b, PI. I) is at first 

 somewhat separated from the Wolfiian duct, but ■ approaches 

 close to it in No. 3. In Nos. 2 and 3 there appears a rod-like 

 body on the outer side of the walls of the groove. In No. 2 



