no 



CLEAVAGE AND DIFFERENTIATION 



The AB blastomere, if isolated, produces a larva which lacks the 

 apical organ and the region of the body behind the main ring of 

 cilia (the post-trochal region), including the coelo-mesoderm. But 

 the apical organ and post-trochal region are present in larvae de- 

 veloped from isolated CD blastomeres : these structures are, how- 



Fig. 52 



A, B, 4-cell stage, later cleavage in Dentaliuni, vegetative views. A, The polar lobe 

 has been retracted. B, It is protruded again but of smaller size than earlier 

 (fig. 5 1) in preparation for the third cleavage. C, 8-cell stage : polar lobe retracted 

 into iD. D, fourth cleavage: iD divides into 2D and a 2^ cell (first somatoblast) 

 containing polar lobe material. E, i6-cell stage. F, 32-cell stage, showing 

 formation of third quartet of micromeres. (After Wilson, from Morgan, Ex- 

 perimental Effibryology, Columbia University Press, 1927.) 



ever, of normal full size, and therefore disproportionately large for 

 the half-sized larva ^ (fig. 53). 



At the approach of the second cleavage, the polar lobe is pro- 

 truded again from blastomere CD, and becomes incorporated into D. 

 If blastomeres A, B,or C are isolated, they resemble AB in that the 

 larvae into which they develop lack the apical organ and the post- 

 trochal region. These structures are present, but relatively much too 

 large, in the miniature larvae developed from isolated D blastomeres. 



^ E. B. Wilson, 1904 a. 



