18 OUTLINES OF CHORDATE DEVELOPMENT 



epithelial arrangement of the cell layers (Fig. 6, A) and com- 

 plicates somewhat the otherwise simple process of gastrulation. 

 The transitional region between ectoderm and endoderm is the 

 blastopore. The region of rapid cell multiplication, i.e., the 

 germ ring, is, therefore, described as located in the lip of the 

 blastopore, chiefly toward the dorsal side (Fig. 6, B-G). 



After the cells within the dorsal lip of the blastopore have 

 partly folded in, the flattened plate of large endoderm cells of 

 the vegetal pole swings gradually inward without undergoing 

 much bending or arching, as if hinged at the lower lip of the 

 blastopore (Fig. 6, D). This process involves a much greater 

 extent of motion in the dorsal margin of this plate, and this is 

 made possible by the rapid multiplication of cells in the dorsal 

 margin of the blastopore, a sheet of which is left behind as the 

 endoderm cells swing inward. The obliteration of the blastocoel 

 continues gradually toward the ventral margin of the blasto- 

 pore, and though finally completed, for a long time a trace of 

 the cavity may be seen in the ventral lip. Occasionally the 

 entire blastocoel may be obliterated completely in a very early 

 stage but this is not a typical condition. 



From this description of invagination it is evident that the 

 endodermal or inner layer cells are really of two kinds, first, the 

 large deutoplasmic cells of the vegetal pole of the blastula 

 forming the ventral region of the endoderm, and second, the 

 smaller cells from the dorsal region of the transitional zone 

 between the animal and vegetal poles of the blastula, which 

 come first to lie in the dorsal lip of the blastopore, and then by 

 rapid multiplication bud off a sheet of cells forming in general 

 the dorsal region of the endoderm. This group of more active 

 cells really lies just outside the margin of the blastopore so 

 that the cells contributed by it to the endoderm are really 

 turned in or inflected after their formation and a very brief 

 existence as ectodermal cells (Fig. 6, D, E). This inflection 

 (involution) of the marginal cells is of importance in comparing 

 the gastrula of Amphioxus with that of other forms. 



But the cells formed by divisions in the dorsal blastoporal 

 lip (germ ring) are by no means all added to the endoderm layer. 



