No. 2.] THE DEVELOPMENT OF THE NEWT. ■^t^'j 



first at the ends of the mid-line and remains exposed longest 

 in the middle. (Fig. 38.) 



There are, however, important variations in the process above 

 outlined. Such a one is depicted in Fig. 37. Here, it will be 

 observed, the closure has been chiefly from behind forwards as 

 indicated by the faint groove running off posteriorly. Sections 

 through the region of this groove show a fusion of the layers, 

 and I have found by the reconstruction of embryos in this stage 

 that this fusion of layers is of greater extent than the fusion 

 at either the anterior or the lateral lips. This condition is 

 comparable with that described by Robinson and Assheton ('9i) 

 in the eggs of Rana tcniporaria. In the newt the posterior 

 groove seems to be more clearly marked than in Rana and the 

 visible remnant of the yolk-plug is oval rather than circular in 

 outline. I agree with these authors in believing that in eggs 

 showing such a structure there has been a coalescence of the 

 posterior lips of the blastopore, but I cannot regard this as 

 illustrating the "typical" method of closure for the blastopore 

 of the newt. It must, I think, be considered rather as the 

 usual preliminary to a stage similar to that shown in Fig. 38, 

 though I have unfortunately been unable to determine this 

 point in the living o-gg. I am certain that in the newt the 

 whole blastopore rarely, if ever, closes by the progressive coa- 

 lescence of the posterior lips alone, although I admit that such 

 posterior coalescence does sometimes anticipate to a greater or 

 less degree the movement of the anterior cells. The simul- 

 taneous approximation of the lateral cells botJi anteriorly and 

 posteriorly, as shown in Fig. 38, is the more usual occurrence 

 in the newt. 



Hertwig ('82, PI. XII, Figs. 3 and 4) has figured stages in the 

 closure of the blastopore of Triton tcsniatns that agree very 

 closely with what I have observed in Diemyctylus. Fig. 4 

 (Hertwig, '82) shows the lines of closure stretching off both in 

 front of and behind an oval yolk-plug. I infer that this must 

 indicate the same general method of antero-posterior closure 

 which occurs in Diemyctylus. Morgan ('89, PI. XLII, Fig. A) 

 also has figured a similar stage for Amblystoma, but attributes 

 the appearance (p. 356) to the elongation of a circular blasto- 



