No. 2.] EARLY DEVELOPMENT OF AMBLYSTOMA. 371 



tions but a single layer of cells. The dorsal lip (d.l) of 

 the blastopore is well defined, the ventral being indicated 

 by a slight thickening {g.r) at the opposite side of the egg. 

 At the dorsal lip in the slight angle formed by the in- 

 folded epiblast there are a few loosely scattered mesoblastic 

 cells {m). 



Passing now to Fig. 3, which represents a vertical section 

 along the line xy of PI. XX, Fig. 3, we observe a decided 

 differentiation of the mesoblast l^nes) even before the infold- 

 ing is well marked. These mesoblastic cells, in size, pigmenta- 

 tion, and reaction to stains, more closely resemble the cells of 

 the epiblast than those of the entoblast. 



In sections (Fig. 4) through a still later stage (between 

 Figs. 3 and 4, PI. XX), the cleavage cavity is much smaller, 

 having been gradually reduced as the gastral cavity has extended. 

 The roof of the gastral cavity is made up of two kinds of cells, 

 which merge along a fairly well defined region. The cells 

 nearer the blastopore partake of the character of the epiblastic 

 cells, while toward the segmentation cavity they resemble those 

 of the entoblast. The thin anterior wall of the mesenteron, as 

 well as the communication often found between the two, lead 

 one to infer that a part of the segmentation cavity may often 

 become continuous with that of the mesenteron. 



In the angle formed by the invaginated hypoblast the meso- 

 blast {mes^i is better defined. Some distance below the dorsal 

 lip of the blastopore is a second group, which is the ventral 

 portion of the ring (ventral mesoderm). 



There is, then, at even this relatively late stage, a continu- 

 ous layer of mesoblast around the blastopore. Soon a division 

 occurs at the dorsal lip, due possibly to the pressure of the 

 chorda hypoblast against the epiblast. Whatever may be the 

 cause, the fact is, that before the blastopore is entirely closed 

 there is no longer mesoblast in this region, while it is present 

 and well defined on either side of the chorda. 



The later growth of the mesoblast is through the antero- 

 lateral extension of the wings lying on either side of the chorda 

 and the postero-lateral extension of the continuous zone at the 

 ventral lip of the blastopore. 



