o-o JORDAN. [Vol. VIII. 



The process of mesoblast formation in the extreme anterior 

 end of the embryo is of a particularly interesting nature; the way 

 in which it here develops is represented in Fig. 64. Here the 

 archenteron is in free communication with diverticula of meso- 

 blast on either side. The mesentoblast in this locality is in a 

 similar condition in all eggs of this stage that I have examined 

 although it rarely presents such a strikingly pouched arrange- 

 ment as is shown in the section I have figured. I have 

 purposely illustrated the clearest case of mesoblastic diverticula 

 that has come under my notice, but the same general arrange- 

 ment of cells, although often masked and obscure, exists always 

 in this region and almost always suggests diverticula from the ar- 

 chenteron. That such a connection of mesoblast and entoblast 

 indicates an origin for the mesoblast of the newt similar to that 

 typified by the serial gut-pouches of AnipJiioxiis seems to me 

 probable. Such a condition of the mesoblast as exists in the 

 newt tends unmistakably to support Hertwig's well-known 

 coelom theory. Fig. 59 shows a similar state of mesentoblast in 

 the hinder part of the embryo although here the pouches are 

 less pronounced. I have not been able to discover any very 

 clear serial arrangement of these archenteric diverticula, but 

 hesitate to say that such metameric arrangement does not exist. 

 If present at all in the newt, however, it is decidedly obscure. 



The mesoblast, then, appears first around the margin of the 

 blastopore and is consequently in this region in close connection 

 with the entoblast. This connection of mesoblast and entoblast 

 is ruptured somewhat irregularly and is usually broken first near 

 the middle of the embryo. Precisely as in the case of the 

 entoblast I have not found it possible to distinguish between 

 the mesoblast derived from invagination and that derived from 

 the yolk-cells. This connection of mesoblast and entoblast is 

 confined to a limited area on either side of the dorsal median 

 line ; laterally the two layers are not in contact. The mesoblast 

 appears to originate chiefly, if not solely, in this locality (Figs. 

 59, 60), and to proliferate ventrally. In later stages it com- 

 pletely encircles the yolk in the hinder part of the embryo 

 (Fig. 56), but more anteriorly (Fig. 57) the sheets of mesoblast 

 are still separate and do not yet meet in the median ventral 



