ON THE FORMATION OF THE GERMINAL LAYERS IN PETROMYZOX. 137 



cells, iiitcrNciiiiii^' Ijctwecn yolk cells and the epiblast, and stretching' 

 not only ventrally but also doryally. 



In enibry(j:3 a little more advanced, these relations are much more 

 clearly brought out. Fig. 7 is the median sagittal section of sucli an 

 embryo. A\ e again find the mass of small cells budded out from cells 

 forming the floor of the invaginated tulje for a short distance from the 

 entrance. 



Figs. 8-1 1 are a series of frontal sections from an emljryo of about 

 the same stjige. Fig. 11 is the most posterior representecL It passes 

 through the line ah in fig. 12. i.e., just through the ventral li[) of the 

 blastopore. Here tlie mesoblustic mass which is distinguished l)v the 

 small size of its cells and the intensive staining of their nuclei, is not 

 paired but is continuous across, being interposed between the epiblast 

 and the yolk-cells. The median j)ortion <3f it is continuc^us with the 

 epiblast and reminds us strijngly of a section through the primitive 

 streak of hifj'her vertebrates, with which it is no doubt homoLjo-ous. 

 Fiu'. 10 shows the section i);issini>" throui>'h the line cd in hu'. 12. 

 Tlie mesoblastic mass is on the whole paired but still continuous 

 across the median line, as the section is not vet out of the region 

 wliere mesoblast cells nre lacing budded out from the floor of the 

 invaii'inated tube. Fiü'. 9 is throuHi the line ef in fi"-. 12. The 

 mesoblastic mass is distinctly paired in this section, and [)asses in front 

 int(j the ufastral mesoljlast in fig. 8. The.se facts are suiBcient, I 

 think, to justify my conclusion stated above, viz : that mesoblnst cells 

 are budded out from all around the lip of the blastopore except the 

 dorsal median point. 



V). The gastral Mesohlaal . In the region of Ûïq body anterior to 

 the blastoporic opening, the mesoblast arises as paired masses, one on 

 each side of the c\n)V(\\\.- anläge. The manner in which this mass is 

 formed, is fundamentally the same througliout every portion where 



