The Fate of the Peristomal Mesoderm and the 

 Tail in Petromyzon 1 ' 



By 



S. Hatta. 



Except my two papers (91, 07) we have had no publication which 

 deals with the peristomal mesoderm in Petromyzon. Basing upon his 

 observations in Amphibia, Mollier (06) asserts that in Petromyzon that 

 colossal mass of vascular cells which appears under the enteric canal, is 

 derived from the yolk cells-* and from a portion of embryonic tissue which 

 I have identified (91) with Rabl's peristomal mesoderm in Selachii. 



The assumption of Mollier is, however, not correct. The vascular cells 

 are derived from certain parts of the mesoderm established (Hatta, 14a ; 

 Keiser, 14). No part of the peristomal mesoderm is resolved into the 

 vascular cells, but it represents a temporary stage by which the micromeric 

 layer round the blastopore develops into the gastral mesoderm. To make 

 intelligible the accounts given in the following pages, the process, about 

 which the peristomal mesoderm is called in existence, will briefly be stated 

 below. 



During the invagination the blastoporic lip is raised into a ridge 

 describing an arc which is most prominent at the mid-dorsal point of the 

 lip and grows lower towards both the arms of the arc (Hatta, 07, fig. 7c.) 

 which occur along the lateral lips. On sagittal and frontal sections it is 



1) Lampetra mitsukurii, Hatta. 



2) In spite of their entodermic origin, the cells of this lot are regarded by the author as the 

 mesodermal. According to his assumption, the free cells have destination to form the ventral 

 portion of the mesoderm, but they are directly converted into the vascular cells, jumping 

 over the process, by which the epithelial mesoderm is produced. 



