Development ot" the Kxcretory Orfjans of ndellostom» .stouti rjockiiifiton. 213 



ame time the pockets have become deeper, and, since the fusion has 

 ap})roached the tubules, they have also become much narrower. This 

 is illustrated by the series (Figs. 8 a — 8 g) from the thirty-tifth segment 

 of the embryo first described. The pocket is here present in only 

 two sections (Figs. 8 c and 8d). 



It was stated above that on one side, in the posterior region, 

 there were places where the duct was still connected with the coelomic 

 epithelium ; here the pockets can hardly be said to exist, but throughout 

 the greater part of the system they are well developed. 



In some cases, as in Fig. 10, taken from about the middle of the 

 region corresponding with the mesonephros of the adult, and from a 

 different embryo, the pocket extends inwards a little distance lieyond 

 the opening of the tubule, so that there is here a part of the somatic 

 layer of coelomic epithelium between the inner angle of the pocket 

 and the opening of the tubule. The same is seen also in Fig. 8 d, 

 although to a much less extent. 



As has already been said , some of the tubules have lost their 

 connection with the general body-cavity. This has been effected by 

 the somatic and splanchnic layers of the coelomic ei)ithelium meeting 

 and fusing with each other just beyond the opening of the tubule 

 into the coelomic pocket, and consequently, not only the tubule, but 

 also the pocket with it, has become separated from the general body- 

 cavity. Later stages show that the coelomic pocket forms an essential 

 ])art of the mesonephric tubule of the adult; the cavity itself forms 

 the cavity of the Malpighian corpuscle, the portion of the somatic 

 epithelium betw'een the inner angle of the pocket and the tubule, 

 together with a part of the wall of the tubule itself, forms the covering 

 of the glomerulus, and the splanchnic epithelium on the floor of the 

 pocket forms the Bowman's capsule. 



At this stage there are no glomeruli. There are blood vessels 

 in the splanchnopleure in such a position that it was at first thought 

 they might correspond to the glomeruli which have been described by 

 RüCKERT ('88 j for the Selachians, and by Boveri ('92) for Amphioxus ; 

 but they do not seem to have any particular relation to the openings 

 of the tubules, nor have they any direct connection with the aorta. 

 It is possible that younger embryos might show different relations. 



A comparison with the adult, and more particularly with the 

 oldest embryo, makes it possible to determine the position where the 

 parts which have been described as pronephros and mesonephros in 

 the adult will come to exist; but a careful study fails to show any 



