ar 



THE EARLY DEVELOPMENT OF THE FROG 101 



once having passed the equator begins to narrow as a whole 

 (i.e., diametrically), and this is chiefly accomplished by the 

 drawing in of its lateral regions toward the mid-line posteriorly. 

 This, together with the very active multiplication of its con- 

 stituent cells, causes this portion to push down more rapidly 

 than the remainder, carrying along the layer of invaginating 

 endoderm, and increasing considerably the vertical extent of 

 the archenteron. Surface views show that at the same time 

 the archenteric groove extends laterally, becoming first cres- 

 centic, then semicircular and finally circular (Fig. 35). That 

 is to say, the first invagination of the pigmented cells forms the 

 dorsal lip of the blast opore; then the invagination gradually 

 extends laterally in each direction forming the lateral lips of 

 the blastopore; and finally the process of invagination is carried 

 around to the side of the gastrula, almost diametrically oppo- 

 site to that where it began, and forms there the ventral lip of 

 the blastopore, and the circular blastoporal margin is thus 

 completed. During the completion of the blastoporal rim the 

 germ ring has continued to extend downward over the yolk on 

 all sides, so that by the time the rim is completed by the forma- 

 tion of the ventral lip, this is found at a level quite below that 

 at which invagination began on the dorsal side (Fig. 35). The 

 invagination involves the inturning of the cells transitional 

 between the animal and vegetal poles, so that the cells of the 

 pigmented and white areas are brought into sharp contrast. 

 The circle of white yolk cells left within the blastoporal rim is 

 called the yolk plug (Figs. 22, B; 32, E, F; 33, C). As the rim 

 draws together, i.e., as the blastopore closes, the yolk plug 

 appears gradually to diminish in size, while it really draws 

 within, or some would say that it is pushed within, by the over- 

 growing lips of the blastopore, until finally it is no longer visible 

 upon the surface (Fig. 38, B). The blastoporal opening then 

 remains as a narrow elongated slit leading directly into the 

 archenteron. 



The precise way in which the germ ring narrows, or as we 

 ight say, in which the blastopore closes, is a matter of some 

 importance. It has already been stated that the dorsal lip 



