No. 2.] THE DE]'ELOPMENT OF THE NEWT. 351 



line. This condition is probably due to the fact that the an- 

 terior mesoblast develops more tardily than the posterior, and 

 is therefore really at an earlier stage in Fig. 57 than in Fig. 56. 



The mesoblast in the newt has in this sense, then, a double 

 origin. It arises like the entoblast in part from the blastopore 

 rim, and in part from the yolk-cells along either side of the 

 median dorsal line. Like the entoblast, too, the mesoblast 

 from these two sources is separated by no well-marked line, 

 but to all appearances is completely continuous. It is, from 

 the outset, however, separated by the chorda-entoblast along the 

 median dorsal line into two lateral sheets, and consequently may 

 be said to have along the greater part of its course a paired origin. 



The origin of both entoblast and mesoblast, therefore, jDoints 

 to a fundamental unity of origin for both layers, and indicates 

 that the distinction of mesoblast as "gastral" or "peristomal," 

 although justified perhaps by considerations of convenience, is 

 not grounded on an essential difference. Instead of regarding 

 the mesoblast as having in reality a " double origin," I think 

 we may suppose that the splitting off from the entoblast of 

 the lateral mesoblastic leaves is simply a continuation into the 

 embryo, as it were, of the process of invagination. It is pos- 

 sible that the two modes of mesoblast formation were originally 

 alike and that differentiation has stepped in only secondarily. 

 The continuity of entoblast and mesoblast is certainly a point 

 in favor of this view, as is also the progressive formation of 

 mesoblast from behind forwards. It is also significant that 

 the mesoblastic diverticula are, as in AmpJiioxus, most pro- 

 nounced in the anterior part of the embryo (Fig. 64). 



NotocJioi'd. — The notochord in the newt is, along most of its 

 length, exclusively entoblastic. The early stages of the noto- 

 chord near the middle of the embryo are shown in Figs. 53, 

 54. The cells of the median dorsal wall of the archenteron 

 assume a somewhat columnar form, and are gradually pushed 

 up and pinched off until they are completely separated from 

 the entoblast and come to lie above it in the mid-line. The 

 gap left in the wall of the archenteron is soon closed up by 

 entoblast cells, and all trace of the line of origin of the noto- 

 chord is thus obliterated. The size of the notochordal cells 



