No. 2.] THE DEVELOPMENT OF THE NEWT, 349 



62, etc.). The entoblast cells which line the archenteron come 

 in later stages to assume a columnar form (Figs. 57, 62). 



I have not been able to discover a stage where I could dis- 

 tinguish only two germ-layers. At the time of beginning 

 invagination there is always an indifferent fused mass of cells 

 around the blastopore rim. It is from this indifferent region 

 that the mesoblast is first formed. The line between ectoblast 

 and entoblast at this point must always be an arbitrary one, 

 since individual cells, which at one time lie externally, are a 

 few hours later rolled under the edge and thus converted into 

 entoblast. Whether we shall regard the mesoblast in this re- 

 gion as coming from the inner or outer germ-layer is largely a 

 matter for personal predilection to determine. It is in a sense 

 true that the mesoblast comes from both layers since it un- 

 questionably develops in just that indifferent, indeterminate 

 region where ectoblast is being continually transformed into 

 entoblast by the rolling under of the outer cells. It has 

 on the whole, however, a much more intimate relation 

 with the invaginated cells than with the cells just on the 

 outer edge. 



The mesoblast grows forward in this way from the blastopore 

 rim and has, even in very early stages, a greater forward exten- 

 sion dorsally and ventrally than on the sides. Precisely how 

 much of this is due to forward proliferation and how much to 

 differentiation in situ I have not been able to determine. Ex- 

 cept for the comparatively limited fused area immediately in 

 front of the blastopore, the mesoblast is deficient along the 

 median dorsal line (Figs. 54, 55, 59). In the immediate neigh- 

 borhood of the dorsal line and at a somewhat variable distance 

 in front of the region of fusion the mesoblast comes into close 

 and unmistakable relations with the entoblast. The mesoblast 

 exists here in the form of small, scattered cells which can in 

 some cases be sharply distinguished from the entoblast cells, and 

 in other cases run into them by imperceptible gradations. Fig. 

 59 shows one of these localities where mesoblast and entoblast 

 are differentiating from the same mass of cells. This conversion 

 of yolk-cells into mesoblast appears to take place first in the 

 hinder part of the embryo and progresses anteriorly. 



