EMBRYOLOGY 



489 



the exposed area of the latter. This extension of. the, epiblast is due 

 to the division of the cells at the circumference of the white area 

 in such a way as to separate small epiblast cells outside from large 

 cells inside which belong to the primitive hypoblast or endoderm, 

 the epiblast cells becoming pigmented as they form. Thus a skin 

 of epiblast is, as it were, cut off 

 from the surface of the yolk cells. 

 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 epiblast over a solid mass of 

 hypoblast without an enteron. 

 But on one side of the white 

 surface, just below the edge <jf 

 the epiblast, there appears a 

 small, shallow, crescentic slit, 

 with its convex edge towards the 

 black area. Where the advancing 

 epiblast reaches this, the process 

 by which it is extending changes 

 and is converted into a multipli- 

 cation of the cells of the convex 

 side of the slit, so that a fold or 

 lip grows and projects over the 

 yolk cells on the other side of the 

 slit and a narrow space is enclosed 

 between the arched lip and the 

 yolk cells. This space is the 

 enteron. The cells on the outer 

 side of the lip are of course 

 epiblast, continuous with the rest 

 of that layer. The cells of the 

 inner side or lining of the lip 

 are small hypoblast cells and 

 form the roof of the enteron, 



Fig. 369. — Sections of a frog's 

 egg in successive stages of 

 gastrulation. The sections 

 are vertical and pass along 

 the future longitudinal axis 

 of the body, the hind end 

 being to the right of the 

 figure. 



A, B, and C correspond roughly to A , 

 B, and D of Fig. 363. 



blc., Blastoccele ; end., yolky endoderm 

 cells ; end',, small endoderm cells ; 

 int., gut ; y.p., yolk plug in blasto- 

 pore. 



3 2 



y p. 



