49° MANUAL OF ELEMENTARY ZOOLOGY 



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

 growing. The lip is the upper edge of the blastopore, the rest of 

 whose edge is as yet indefinite and represented by the limit of the 

 advancing epiblast all round the egg. All this time the shape of the 

 crescent is changing by its two ends lengthening and curving towards 

 one another till at last they meet to form a circle. By that time the 

 edge of the epiblast has reached this circle all round its circumference, 

 so that all the yolk is covered except that within a circular area, the 

 definitive blastopore, bordered by a continuous lip and filled by a. yolk 

 plug consisting of yolk cells which have not yet been covered. The 

 lip continues to grow over the yolk plug, thus narrowing the 

 blastopore. The narrowing, however, takes place not by ingrowth of 

 the lip all round, but by the growing 

 *"■'■ together of the sides of the circle in its 



hinder part. Where the sides thus 

 coming together meet, there remains 

 a seam in the form of a groove — the 

 primitive groove — under which lies a 

 band of cells — the primitive streak — 

 in which epiblast, hypoblast, and meso- 

 blast meet and fuse. Finally, the plug 

 is covered and the blastopore is a minute 

 opening at the bottom of a slight de- 

 pression, from which the primitive 

 groove runs backwards. During the 

 Fig. 370.— The embryo of a later sta g e s of this process an internal 

 frog shortly after the com- movement of the yolk cells has obliter- 

 pletion of gastrulation, ated the blastoccele and enlarged the 

 seen from the right side eriteron, which was at first a mere slit, 

 and somewhat from be- s0 . that ll becomes a spacious cavity, 

 h m( j. which communicates with the ex- 



terior by a slit between the dorsal 

 «A, Blastopore ;«./, neural folds. s ide of the blastopore lip and the 

 yolk plug. 



At the end of gastrulation the enteron is a large cavity 

 with a very thick ventral wall composed of 

 Me™bia Bt.° lar S e yolk cells, many deep, and a thinner 

 dorsal wall composed of smaller and fewer cells. 

 From this wall, which is the primitive hypoblast, the meso- 

 blast, or embryonic mesoderm, and the notochord have 

 already begun to separate, leaving the hypoblast proper 

 (Fig. 371). The mesoblast arises by the splitting off of an 

 outer layer of cells. The split starts on the dorsal surface 

 on each side of the middle line, in the same position in 

 which the pouches arise in Amphioxus, and spreads outwards 

 around the thick ventral wall of the gut. For a time the 

 mesoblast remains connected with the hypoblast along the 



