LATE CLEAVAGE THROUGH GASTRULATION 



17 



10 



24 



28 



LATE CLEAVAGE 

 MACROMERES 



MANY SMALL CELLS 

 EARLY BLASTULA 

 LARGE CELLS 



LATE BLASTULA 



EARLY GASTRULA 



DORSAL LIP OF BLAS- 

 TOPORE 

 YOLK CELLS 



MIDDLE GASTRULA 

 LATERAL LIPS FORM- 

 ING 

 YOLK CELLS 



LATE GASTRULA 



LIPS OF BLASTOPORE 



COMPLETE 



VENTRALLY 



Fig. 58. Stages of development of Rana 

 sylvattca from 6 to 28 hours. The period is 

 characterized by a tremendous increase in 

 number of cells and a rearrangement of cells 

 comprising the phenomenon of gastrulation. 

 Between stages 7 and 8 the cells become so 

 small that it is no longer possible to repre- 

 sent all the individual cells at this magnifica- 

 tion. The cells at the vegetal pole are many 

 times larger than those at the animal pole 

 and thus are pictured in the drawings of 

 stages 8 and 9. By stage 10, even these vege- 

 tal cells are too small to be illustrated, and 

 they are represented by light stippling in 

 stages 10, 11, and 12. At stage 10 a dark 

 line of pigment appears on one side of the 

 egg. This is the first sign of gastrulation. It 

 means that some cells have changed shape 

 and are migrating internally. An overgrowth 

 of the white yolk by the pigment cells ac- 

 companies the internal migration. The over- 

 growth results in a sharp line between pig- 

 mented and white cells which marks the 

 blastoporal lip. First there is simply a dorsal 

 lip of the blastopore (stage 10) ; then lat- 

 eral lips form (stage 11); and finally the 

 ventral lip completes a circular blastopore, 

 which is plugged with yolk cells (stage 12). 

 By this means the yolk cells are completely 

 covered by the pigmented cells. (From Pollis- 

 ter and Moore, Anatomical Record (1937), 

 68, 489.) 



stage 7 and 8 (Fig. 59) show the size differences among cells at stage 7 and 

 the great increase in number of cells at stage 8. 



During the later stages of cleavage an internal cavity, the blastocoel, 

 forms. It is located mainly in the animal hemisphere (Fig. 62). About 19 

 hours after fertilization the dorsal lip of the blastopore appears as a dark 

 line just below the equator of the egg (stage 10, Figs. 58 and 60) . Cells begin 

 to migrate internally and the archenteron develops. The archenteron is very 

 small at first but with the continued migration of cells into the interior and 

 the overgrowth of cells over the vegetal hemisphere the archenteron increases 



