THE HEREDITY OF 8EX 43 



superficial cytoplasm which remained alive, and 

 this proceeded to develop that particular part of 

 the embryo to which it would have given rise if 

 the rest of the egg had not been killed. There was 

 no regeneration of the part killed, no formation 

 of a complete embryo. It may be pointed out 

 that segmentation in the insect egg is peculiar. 

 The nuclei multiplied by segmentation migrate 

 into the superficial cytoplasm surrounding the yolk, 

 and then this cytoplasm segments, and each part 

 of the cytoplasm develops into a particular region 

 of the embryo. This, of course, does not prove 

 that the nuclei or their chromosomes do not de- 

 termine the characters of the parts of the embryo 

 developed, but they show that the parts of the 

 non-nucleated cytoplasm correspond to particular 

 parts of the embryo. The most important object 

 of investigation at the present time is to find the 

 origin of these properties of the chromosomes. We 

 may say, using the word < determinant ' as a con- 

 venient term for that which determines the adult 

 characters, that in order to explain the origin of 

 species or the origin of adaptations we must discover 

 the origin of determinants. Mendelism does not 

 throw any direct light on this question, but it 

 certainly has shown how characters may be inherited 

 as separate and independent units. When one 

 difference between two breeds is considered, e.g. 

 rose comb and single in fowls, and individuals are 

 crossed, we have the determinant for rose and 

 the determinant for single in the same zygote. 

 The result is that rose develops and single is not 

 apparent. In the next generation rose and single 

 appear, as at the beginning, in separate individuals. 



