224 



INTERNAL FACTORS 



IV. i 



micromeres is formed from the clear animal area ; after this 

 division the polar lobe of D moves up to meet the upper white 

 area, and so it comes about that a large part of this lobe passes 

 into the second micromere, 2 d } which is the first somatoblast. 

 In the other quadrants the second micromeres are formed from 

 the upper white area alone. All the cells of the third quartette 

 and (it is not known certainly whether this is the mesoblast) 4 d 

 in the fourth are quite white. The larva is spindle-shaped with 



d e f 



FIG. 134. Dentalhtm. Cleavage after removal of the first polar lobe ; 

 a, first division: the isolated polar lobe is seen below; b, 4-cell 

 stage, from animal pole ; c, 8-cell stage, from animal pole ; d, forma- 

 tion of second quartette, from vegetative pole ; e, same stage as last, 

 open type ; /, same stage, from animal pole. (After Wilson.) 



a broad, triple prototroch. There is an apical organ of long cilia 

 and a tuft of sensory hairs at the opposite end (Fig. 135 }. 



When the polar lobe is removed in the ' trefoil ' (2-celled) stage, 

 segmentation continues, but no polar lobe is formed in any stage, 

 the D cells are not larger than those in the other quadrants, the 

 embryo has no lower white area, the segmentation cavity may 

 be open below, and the larvae are deficient and die (Figs. 134, 

 135 li]. Post-trochal region and apical organ are both absent and 



