234 



THE CONTINUITY OF THE RACE 



tied cleavage, blastula and gastrula of the frog (Fig. 15.15) and the highly 

 modified development of the chick (Fig. 15.16). 



Cleavage. Soon after fertilization has taken place, the zygote divides 

 by mitosis to form a 2-cell embryo. The plane of the first division, or 



arch. 



Fig. 15.14. The early development of the lancelet Amphioxus up to the formation of noto- 

 chord, neural tube, and somites. A, the fertilized egg. B, 2-cell stage. C, 4-cell stage. D, 

 8-cell stage. E, 16-cell stage. Fi, 32-cell stage. F2, 32-cell stage, optical section (as are all the 

 figures H to N). G, morula. H, blastula. /, flattening of vegetative pole. «/, early gastrula. 

 K, gastrula. L, late gastrula. M, embryo at neural fold stage. N, embryo at neural tube 

 stage, ap, animal pole; arch, archenteron; bl, blastocoel; bp, blastopore; ect, ectoderm; ent, 

 entoderm; mf, mesodermal fold; n, nuclei; rich, notochord; ncl, neural canal; npl, neural 

 plate; nt, neural tube; pb, polar body; som, somites; vp, vegetative pole. (Modified from 

 Turtox chart, courtesy General Biological Supply House, Inc.) 



first cleavage, passes through both poles of the zygote and divides it into 

 approximately equal cells. The next cleavage also passes through both 

 poles of the embryo, at right angles to the first cleavage plane, and divides 

 each of the 2 cells to form a 4-cell stage. In the next division each of the 

 4 cells divides in a plane at right angles to the first and second cleavage 



