622 



embryology: lancelet 



the blastula is converted into a two-layered structure, thegastrula 

 (Fig. 480, 7). This begins b}^ a flattening of the dorsal area of yolky 

 cells. Next these are dimpled or invaginated into the blastocoele, 

 forming a cup which, in spite of the difference in shape, may be 



— Wc 



Fig. 480. — The gastrulation of Bvanchio stoma. — After Conklin, with modification, 



I, Sixty-four cell stage ; 2, completed blastula, from the left (and somewhat from the ventral side, so that 

 the mesoderm appears rather more dorsal than it is) ; 3, approximately the same stage, from the ventral 

 side and a little from behind ; 4, right half of the same viewed from within ; 5, beginning of gastrula- 

 tion ; 6, later stage ; 7, late gastrula ; hlc, blastocoele ; Up., blastopore ; d.l., dorsal lip ; ec, ectoderm ; 

 en., endoderm ; w., mesoderm. 

 The side which will be anterior in the adult is to the left of each figure : the dorsal side is shown with the 



exception of 2, which is a ventral view. 



compared with the body of a Hydra. The yolky cells — the endo- 

 derm or hypoblast — form the lining while the mouth or blastopore. 



