MORPHOLOGY OF CYCLOSTOMATA. 353 



well as the coelomic projection ir.///.) is very much decreased in 

 thickness and encloses the peritoneal cavity [pp.c.) that has now 

 become spacious, while the visceral peritoneum is still thicker than 

 other parts. The mesenchymatous cells found in the foregoing stage 

 on the median ventral line are transformed into a thin layer of the 

 endocardium of the heart and its anterior continuation {h. and tr.a.) 

 which are suspended by the dorsal and the ventral raensenteries, 

 and enclosed in the thick pericardial coat still forming a con- 

 tinuation of the peritoneum. I can detect, however, none of the 

 traces of the peritoneal partition which was developed so markedly 

 in the hist stage that one could not possibly overlook them. I 

 have endeavoured to trace tlie mode of disappearance of this 

 structure, but have only found that in one lot of embryos the 

 whole set of the structure was present while in the other no trace 

 of it was perceptible. Unfortunately I have found no embryo in 

 an intermediate condition. 



The few cells observed from the last stage beneath the chorda, 

 and also in the space outside the pronephric tubules on both 

 sides are more or less multiplied. As the former group is 

 transformed tinally into the dorsal aorta, and the latter into the 

 anterior cardinal vein of either side, I shall call them the tract of 

 the dorsal aorta and of the anterior cardinal veins respectivel}'. 



Fig. 1)(S represents the section through the hind border of 

 the branchial region. On either side of the enteric tube a small 

 space {pp.c.) of the body-cavity is surrounded by the peritoneal 

 epithelium, still consisting, in this part, of somewhat cubical 

 cells. The ventral edges of the peritoneal membrane of both 

 sides are just meeting at the median ventral line. A few mesen- 

 chymatous cells .{tr.a.) found in the space between this meeting 

 point and the ventral wall of the enteric canal, are destined to 



