GASTRULATION 631 



edge, so that what was on the upper surface comes under to form 

 the archenteron roof. The arrows in Fig. 490 C and C" show the 



nchi 



ep^- 



■Htes. 



-Xend. 



^-emt: 



S- .end. 



■y p 



C ''-nd. 



c 



Fig. 490. — Gastrulation in the frog. 



A B and C are sections in successive stages corresponding roughly io A,B, and D of Fig. 4S9. A' is a 

 ' plan of the presumptive areas in stage A. C and C show the rolling m of cells at the blastopore lip. 



at Archenteron roof formed by growth of the lip ; hk., blastocoele ; end., endodenn : end.', cells of xooi 



■' of archenteron, which will form notochord ; ent., gut ; cp., epidermis ; /., blastopore lip mes. niesodenn ; 



n.p., presumptive area of cells for neural plate ; nch., cells for notochord ; y.p., yolk plug m blastopore. 



direction in which the cells move during this process. The cell 

 material thus transferred from the outside to the inside of the 

 roof of the archenteron during the formation of the gastrula is, 

 as we shall see, the material for the mesoderm and notochord. 



