DIFFERENTIATION, HEREDITY, SEX 



273 



Q 



FIG. 126. Development of the Mollusc, Dentalium, after removal of the 

 "polar lobe." From Wilson. A. Egg twenty minutes after extrusion, and 

 before maturation is completed, showing regional differentiation. B. Section 

 through egg one hour after fertilization, showing the beginning of the formation 

 of the polar lobe. C. Normal eight-cell stage, viewed from lower pole. The 

 polar lobe is the light part of cell D. D. Normal sixteen-cell stage viewed from 

 lower pole. The materials of the polar lobe are now contained in the cell marked 

 X. E. Sixteen-cell stage of egg from which the polar lobe was removed during 

 the first cleavage period. F. Normal trochophore of twenty-four hours. G. 

 Trochophore of twenty-four hours, developed from "lobeless" egg. H. Normal 

 larva of seventy-two hours, showing foot and shell. /. Seventy-two-hour larva 

 from "lobeless" egg. p, polar lobe. 



