590 



INVEETEBEATA 



CHAP. 



masses of new cells formed. Indeed, in all cases where the process of 

 gastrulation has been carefully investigated, as for instance in the 

 egg of Echinus, invagination is preceded by a rapid multiplication of 

 cells, near the spot on the wall of the blastula which is invaginated. 



Fig. 429. — Stages in the gastrulation of the egg of Amphioxus lanceolatus. 

 All the figures represent median sagittal sections. 



A, stage of hat-shaped blastula. B, the invagination of the endoderm well advanced ; note the 

 rounded cells at the dorsal lip of the blastopore. 0, the gastrula has grown in length — the neural plate 

 is clearly marked. D, the upgrowth of the ventral lip of the blastopore has begun, n.p, neural plate ; 

 X, dorsal lip of the blastopore ; Y, ventral lip of the blastopore. 



There is no ground, however, for assuming an inflection of cells round 

 the dorsal lip of the blastopore, indeed, as invagination proceeds we 

 obtain definite evidence that this is not the case. For although at 

 first the nuclei of all the cells of the embryo are alike, being large, 

 clear, and vesicular, with relatively little chromatin, yet soon the 



