MOEPHOLOGY OF CYCLOSTO:\LVTA. 351 



constitutes the earliest fundament of the heart, is detected. The 

 present position of the pronephros, — dorsal to the heart, anterior 

 and dorsal to the liver, and along either side of the chorda, — 

 is retained by it for a coinparatively long period (see fig. 97) ; 

 in later stages, the liver is somewhat shifted backwards, so 

 that the pronephros now comes entirely in front of it (see 

 fig. 115). 



In an older embryo of this stage (figs. 92-96) the median 

 folds of the coelomic projection, the component cells of which are 

 very much flattened out, go in deeper towards the median line 

 to meet with its counterpart on the opposite side. The second 

 tubule (figs. 92 and 93, pt.-J) has become weaker, as a com- 

 parison of these figures with figs. 77 and 79 will show. On the 

 contrary, the tubule of the next pair (fig. 94, pt.S) has much 

 elongated and is bent considerably in dorso-lateral direction, 

 so that we can no longer observe the nephrostome together with 

 the tubule itself on the same section. The following tubule, 

 the fourth (fig. 95, pt.4), is likewise well developed ; the fifth 

 (fig. 96, pt.5) is more or less weak in development as compared 

 with the tubules of the two foregoing pairs. In short, these 

 three pairs (from the third to the fifth) make parallel pro- 

 gress with the development of other structures, for instance, the 

 mesenterial fold or the muscle-segments. This is a fact that 

 is to be observed too in the younger embryo of this stage, as 

 above described. At the present stage, we can find no trace of 

 the tubules beneath the ninth myotome, where the tubules of the 

 sixth pair ought to be found, but only the cross-sections of the 

 collecting duct or the anteriormost part of the segmental duct 

 are seen. 



