-; o 5 JORDA N. [Vol. VIII. 



the answers to these questions involve large theoretical con- 

 siderations. 



In my description of the closure of the blastopore in the 

 newt, I shall, in this section, confine myself chiefly to a dis- 

 cussion of the questions already advanced and shall defer to a 

 later section all extended reference to the origin and develop- 

 ment of the germ-layers. 



Method of closure of the blastopore. — There are several con- 

 ceivable modes by which the blastopore might close. It might 

 close either (i) by a convergence of all the marginal cells towards 

 a central point, or (2) by a sort of peristaltic wave of approxi- 

 mation of the lateral cells from behind forwards or from before 

 backwards, or (3) by a similar growth of the cells dorsally or 

 ventrally, or (4) by a combination of any or all of these processes. 

 Surface views afford most valuable evidence on this point, and 

 for these I have learned to rely chiefly on the study of the living 

 Qgg, since prolonged study of preserved specimens has served to 

 convince me of the confusing nature of the evidence derived 

 from comparison of different eggs in different stages. 



The process in the living egg, as I have observed it in the 

 newt, is usually as follows. When the circle of the blastopore 

 is completed (Fig. 36) there ensues an approximately uniform 

 convergence of cells from all sides. This convergence at first 

 takes place at a nearly uniform rate, as shown by observation 

 of the cell movement at different parts of the blastopore rim. 

 It continues for a very varying length of time, but seems in all 

 cases to be the first step in the dwindling of the blastopore. 

 This method of closure by centripetal convergence of the 

 marginal cells is substantially the method witnessed in some 

 Annelids {Nereis, Rhynchelmis). In the newt, however, such 

 uniform convergence does not continue to take place up to the 

 end. Before the blastopore is obliterated, and usually while its 

 diameter is still considerable, the cells at the ends of a line 

 drawn through the blastopore parallel with the mid-axis of the 

 embryo lag behind the cells at the sides. These latter press 

 rapidly in towards the middle, and the result is the formation 

 first of an oval (Fig. 38), then of a longitudinal slit (Fig. 40). 

 This movement brings it to pass that the yolk-plug is covered 



