100 OUTLINES OF CHORDATE DEVELOPMENT 



ectoderm and endoderm respectively. Hemisection of this 

 very early gastrula (Fig. 32, A), shows that the elevation of 

 the floor of the blastoccel is very rapid at this time, and one 

 of the first results of the arching up of the yolk cells (endoderm) 

 is the formation of a narrow groove all around the margin of 

 the blastocoel, between the base of its wall and its rising floor. 

 As the yolk continues to rise, this groove soon becomes quite 

 marked and compressed into a narrow slit which, though 

 originating in the manner just described, seems to be continued 

 ventrally all around the central cells as a definite splitting or 

 delamination (Fig. 33, A). In effect this narrow space sepa- 

 rates definitely the ectoderm and endoderm in the region 

 within (above) the restricted invaginating region, which of 

 course also gives rise to an ectodermal and an endodermal 

 layer. This original groove is called the gastrular groove, 

 and the delamination which extends it is the gastrular cleavage; 

 the formation of these is not limited to the region of the dorsal 

 lip of the blastopore, but extends entirely around the gastrula, 

 even to the side opposite that of invagination. It remains a 

 question whether the invagination process is the result of an 

 active inturning of the cells forming the lower margin of the 

 germ ring, or whether these cells are rather pulled inward by 

 the elevation of the yolk cells, which results from the com- 

 pression produced by the thickening and downgrowing germ 

 ring; perhaps both factors are involved. However this may be, 

 the invagination once begun continues rapidly, so that an 

 elongating tongue of inturned cells continually pushes up 

 under the superficial ectoderm which lies just above the 

 invaginating region. This tongue is the invaginated endoderm 

 derived from some of the cells of the animal pole and their 

 descendants; at its inner limit it becomes directly continuous 

 with the endoderm formed directly from the yolk cells, or 

 cells of the transitional zone which have become entitled to 

 the name endoderm, while still practically in situ, by the 

 appearance of the gastrular groove and cleavage. 



Turning for a moment to the consideration of the external 

 modifications during gastrulation we see that the germ ring, 



