530 EMBRYOLOGY I FROG 



the outer surface of the blastula, which floats with the black 

 side uppermost. The small, pigmented cells now start to spread 

 downwards, as a skin over the yolk cells, so that the black area 

 which the small cells form increases and the white area where the 

 yolk cells are exposed diminishes (Fig. 489) . This process is known 



as epiboly. If it took place all over the surface 

 of the yolk cells the result would be the 

 formation of a close skin of small cells over 

 a solid mass of hypoblast without an enteron. 

 But on one side of the white surface, just 

 below the boundary, on a greyish crescentic 

 area which arose there in the cytoplasm of 

 the ovum when the spermatozoon entered on 

 the opposite side, there appears at this time a 

 small, shallow, crescentic slit, convex towards 

 the black area (Fig. 489 A IM.). Where the 

 advancing black area reaches this its further 

 extension takes place in a different way, 

 namely, by the upper side of the slit growing out and thus pro- 

 jecting as a Hp-Hke fold over the yolk cells on the other side. The 

 result is that a narrow space is enclosed between the arched lip 

 and the yolk cells (Fig. 490 B). This process, which may be 

 compared to the narrowing of the blastopore of Branchiostoma 



l.bl. 



Fig. 488. — A vertical 

 section of a frog's 

 egg at the end of 

 cleavage. 



hlc., Blastocoele. 



ABC 



Fig. 489. — Stages in the gastrulation of the frog's egg. The egg is seen from the 

 lower, white pole, which faces in the direction of the future hind end of the 

 animal. 



IM., Lip of the blastopore ; y.p., yolk plug. 



by growth of its lip (p. 623), is akin to invagination, not epiboly. 

 The side on which it happens is the future dorsal side of the animal. 

 The space enclosed is the archenteron. The cells on the outer 

 side of the lip are of course continuous with the ectoderm. The 

 cells of the lining of the lip form the roof of the archenteron, its 

 floor being formed by the large yolk cells over which the lip is 

 growing. As the lip extends, the rapid advance of cells on its 

 outer (upper or dorsal) side causes that side to roll over at the 



