MORPHOLOGY OF CYCLOSTOMATA. dû/ 



epithelial structure, forming the continuation of the peritoneum 

 and folding out from the peritoneal cavity. Beneath the first 

 tubule, there is found no rudiment of the projection ; under the 

 second (fig. 59) it is very weak, while beneath the third (fig. 60), 

 fourth (fig. 61), and fifth (fig. 62), tubule, respectively it is most 

 vigorously developed. But on the left side of figs. 61 and 62 it 

 is again in a primitive condition, just as in the last series of 

 sections (figs. 52, 53, 55, and 56). 



The coelomic projections are not confined to the anterior region 

 where the pronephric tubules are found, but it is found likewise 

 in the posterior part where only the segmental duct develops. 

 Fig, 6o show^s the section through the thirteentli somite ; on this 

 section, the duct is cut off from the m3"otome and a well developed 

 coelomic projection {cp.) is observed ; I will return once more 

 to this subject further on. 



Leaving the coelomic projection in this stage of development, 

 I will return to the origin of the Anlage of the pronephros and 

 give somewhat more exact details on the subject. Since the piece 

 of the mesoblast called above the Anlage of the pronephros forms 

 for a time the |)roximal portion of the lateral plate, one might 

 presume that its whole mass wall be transformed into the pro- 

 nephric tulnile and will not partake in the formation of the perito- 

 neal membrane. I was at first of this opinion, l)ut a careful 

 observation of sections through the embryos in each stage showed 

 my error. 



To illustrate this point satisfactorily, I have given, in the 

 annexed wood-cut (Wood-cut 1), a series of semi-diagramatic 

 figures, which show the successive phases of the changes going 

 on in the structure. A shows the first indication of the Anlaue 

 of the pronephros before the separation of it from the myotome ; 



