DIFFERENTIATION, HEREDITY, SEX 



277 



Several other forms are known to give similar results. One 

 of the clearest instances is to be seen in the Lamellibranch, 

 Cumingia, also described by Morgan. The egg -of Cumingia 

 contains, besides the clear protoplasm, three kinds of formed 

 substance, yolk, pigment, and oil. With the centrifuge these 

 can be thrown to any part of the cell whatever, and yet cleavage 

 and development proceed normally with reference to the 



FIG. 129. Normal development of the Pelecypod, Cumingia, following ab- 

 normal arrangement of the egg substances by centrifuging. After Morgan. The 

 pigment is indicated by stipples, the oil by small circles. A. Two-cell stage with 

 oil in small cell. B. Same with oil in large cell. C. Same with oil in both cells. 

 D. Normal trochophore, showing usual distribution of pigment and oil. E. 

 Trochophore with oil on oral side, and yet normal. F. Normal trochophore with 

 oil aboral and interior. 



original polarity and not at all to the actual distribution of 

 these substances (Fig. 129). It should be noted that, although 

 this has not been definitely determined for Cumingia, the 

 Mollusca in general are excellent examples of determinately 

 cleaving eggs and the removal of parts of ova is followed by 

 definitely corresponding defects in embryo and larva. 



Such experiments as these seem to indicate clearly that the 



