380 EYCLESHYMER. [Vol. X. 



dorsal lip. A second portion, forming the gastral mesoblast, 

 arises on each side of the chorda. 



PI. XXI, Fig. 5, represents a vertical section through the 

 area of deepest invagination in an egg, at the stage shown in 

 PI. XX, Fig. 10. Lying between the epiblast and the invag- 

 inated hypoblast is a group of indifferent cells (ni), which forms 

 the basis of the mesoblast. 



A later condition is represented in Fig. 6. The group of 

 cells (jnes) designated (;«.) in Fig. 5, are so highly differenti- 

 ated that they plainly constitute a third layer. This layer 

 extends around the entire periphery of the ^gg. 



The majority of investigators have not recorded the exist- 

 ence of mesoblast at the dorsal lip of the blastopore, yet Scott 

 (■81) has described it as follows: "In der dorsalen Blastoporus- 

 lippe vermehren sich die Mesodermzellen sehr rasch und bilden 

 in der Mittellinie eine continuirliche Platte." 



In Fig. 7, after Hatta, a later stage is shown. The seg- 

 mentation cavity has disappeared, owing to the extension of 

 the gastral cavity. At this time there is a group of cells at 

 the ventral lip of the blastopore {v.mes.), which Hatta inter- 

 prets as the ventral mesoderm, and further states that " meso- 

 blast cells are budded out from all around the lip of the blasto- 

 pore, except the dorsal median point, where the epiblast turns 

 around to join the hypoblast." 



I at once endorse this statement, but would add that in an 

 earlier stage the mesoblast also exists at the dorsal lip; its 

 division at this time is probably due to the pressure of the 

 axial string of invaginated hypoblast. Thus, in Petromyzon, 

 as well as in Amblystoma, there exists a well-defined layer of 

 mesoblast around the entire blastopore. 



In a still later stage (Fig. 8), there is a splitting off of cer- 

 tain cells from the layer forming the roof of the gastral cavity. 

 These form a caudal knob {tel.) to which Kupffer ('9o) has 

 given the name " teleoblast." Kupffer expressly states that 

 the cells take no part in the early formation of the mesoblast. 

 " Ausgangspunkt der Bildung des Mesoderm ist also hier bei 

 P. Plan, nicht der Teloblast, iiberhaupt nicht der Blastoporus- 

 rand." As will be observed from the figures, this stage is 



