DIFFERENTIATION, HEREDITY, SEX 



279 



structure of the egg and the localization of these materials, 

 precede cleavage and are independent of it. Certain pressure 

 experiments to be mentioned shortly, illustrate the independ- 

 ence of cleavage and determination, and the centrifuging 

 experiments similarly demonstrate the independence of 

 cleavage and the distribution of the cytoplasmic stuffs. Indeed 

 Lillie describes the formation of a trochophore-like larva from 

 the egg of Chcetopterus in which cleavage had been artificially 

 prevented; this embryo formed external cilia and certain other 



FIG. 130. Development and differentiation in the absence of cell division, in 

 Chcetopterus. From Lillie. A, B, C. Ciliated, uninucleated unsegmented eggs, 

 about twenty-three hours old. The vacuoles are about in the position of the 

 prototroch of the larva. D. Ciliated unsegmented egg about twenty-eight hours 

 old; most of the endoplasm has been consumed, e, endoplasm. 



differentiated structures in the complete absence of cell divisions 

 (Fig. 130). The normal processes of development are varied 

 and more or less independent of each other, while having 

 common reference to some general underlying condition. 



We must now consider the facts of development in a consider- 

 able group of eggs which do not show any such results as those 

 described in the foregoing pages. Although these eggs possess 

 more or less differentiated regions of cytoplasm, yet removal of 



