n 



48 JORDAN. [Vol. VIII. 



where (pp. 333, 334) indicated my reasons for s tating that 

 this epibolic invagination occurs. 



The roof of the segmentation cavity is for a long time only one 

 cell thick. About the time invagination begins, the nuclei of the 

 roof cells assume an alternate arrangement like that described 

 by Scott and Osborn ("79), and the formation of a double-layered 

 ectoblast proceeds in the way they describe. This doubling 

 is undoubtedly in part due to delamination, and in part 

 also, as Scott and Osborn suggest, to the alternation of wedge- 

 shaped cells which draw in their edges and come to lie one row 

 above the other (Fig. 54). Sometimes the single layer of ecto- 

 blast cells persists to quite a late period (Fig. 55). Whatever 

 the significance of this persistent condition of single-layered 

 ectoblast there is no question that it is radically different from 

 the several-layered condition seen in Anuran embryos of this 

 stage. (See e.g. Schultze's figures of Ra7ia fiisca, '88.) 



The thinning out of the ectoblastic roof of the segmentation 

 cavity and the concomitant thickening near the rim of the 

 blastopore are, I think, genetically related to the corresponding 

 phenomena attending the formation of the germ-ring in Teleosts. 



The entoblast in the newt has in a sense a double origin. It 

 comes in part from invagination of the primitive ectoblast cells 

 and in part from the yolk-cells. Along the dorsal wall of the 

 archenteron it is impossible to distinguish the cells derived 

 from one source from those derived from the other. In such an 

 Qgg as that shown in section in Fig. 5 5 we can safely assert 

 only that the most anterior entoblastic cells have been differen- 

 tiated in siUi and that the most posterior cells have been invag- 

 inated. Between these two extremes there is no cell that we 

 can point to and surely identify as the foremost invaginate cell. 

 The cells derived from the two sources grade so insensibly into 

 each other that the transition point cannot be detected. It 

 does not seem likely that at the early stage shown in Fig. 55 

 the invaginate cells reach as far forward as the middle of the 

 embryo, but such a belief must obviously be grounded on other 

 considerations than those afforded by sections alone. The 

 ventral and a considerable part of the lateral walls of the 

 archenteron are formed from the yolk-cells {cf. Figs. 55, 56, 



