338 JORDAN [Vol. VIII. 



pore. I am unable to convince myself that in the newt there 

 is anything more than a continuation of the process of shrink- 

 age. ^ 



Another not uncommon condition is illustrated in Fig. 39. 

 This is somewhat older than the stage shown in Fig. 38 and is 

 marked by the still circular outline of the very small blastopore. 

 The lines running off behind and in front indicate the seams 

 of closure. 



The closure of the blastopore in the newt, then, may be con- 

 sidered to take place in the following manner. There is first 

 a convergence at a uniform rate of cells from all parts of the 

 margin. This process becomes gradually modified, sometimes 

 very early in the blastopore history, sometimes very late, by the 

 more rapid movement of the cells on either side of the axial 

 mid-line. At this point considerable variations arise, (i) The 

 movement of the posterior cells may anticipate that of the 

 anterior, in which case the blastopore " closes from behind 

 forwards." This occurrence I believe to be quite frequent in 

 the newt. About ten per cent, of the preserved eggs that I 

 have examined seem to be closing in this manner. The point 

 of great interest in these cases, and one which my study of the 

 living ^^^ unfortunately leaves unsettled, is whether a closing 

 also " from before backwards " may not immediately succeed the 

 stage figured (Fig. 37). It is worthy of note in this connec- 

 tion that the blastopore in eggs of the stage showing only a 

 posterior groove is usually of about the same size. When the 

 eggs are in the stage in which the blastopore opening is nearly 

 obliterated there are, I think invariably, lines running in both 

 directions (Figs. 38 and 39). (2) There is the possibility that 

 the movement of the anterior cells may anticipate that of the 

 posterior. This, if it happens at all in the newt, is not common. 

 That it occurs occasionally is, I think, probable. Only in this 

 way can I explain certain puzzling cases in which such a line 

 as shown in Fig. 57 is seen running off miteriorly. (3) The 

 movement of the anterior and posterior cells may be approxi- 

 mately synchronous, in which case the blastopore may be 



1 Dr. Morgan has since informed me that he did not intend to imply actual 

 elongation, but simply the narrowing to an oval of the earlier circular outline. 



