412 



INHERITANCE AND DEVELOPMENT 



be no doubt whatever that four of the cntoblastic nuclei were nor- 

 mally destined for the first quartet of micromeres (Fig. 187, B), from 

 which arise the apical ganglia and the prototroch. Under the condi- 

 tions of the exjXM-iment, however, they have given rise to the nuclei 

 of cells which differ in no wise from the other cntoderm-cells. Even 



Fig. 187. — Modifications of cleavage by pressure in Nereis. 



A. B. Normal four- and eight-cell stages. C. Normal trochophore larva resulting, with four 

 entoderm-cells. D. Eight-cell stage arising from an egg flattened by pressure ; such eggs give rise 

 to trochophores with eight instead of four entoderm-cells. Numerals designate the successive 

 cleavages. 



in a highly differentiated type of cleavage, therefore, the nuclei of the 

 segmenting egg are not specifically different, as the Roux-Weismann 

 hypothesis demands, but contain the same materials even in -the cells 

 that undergo the most diverse subsequent fate* But there is, further- 

 more, very strong reason for believing that this may be true in later 



