THE CELLULAR BASIS 



173 



FIG. 41. NORMAL AND INVERSE SYMMETRY OF THE 3n, 4rH, 

 STH AND GTH CLEAVAGES. The cells la-Id, 2a-2d and 3a-3d give 

 rise to all the ectoderm; 4d or M gives rise to mesoderm; 

 A, B, C, D to endoderm. 



