88 DIFFERENTIATION III 



that the n chromosomes of the germ-cells are unlike one 

 another, yet the entire set contributed by each germ-cell is 

 handed on to every cell in the body, and indeed must be if 

 development is to be normal. The experiment which has 

 just been described suggests that the cytoplasm, different 

 in the various cells of the body, might incite to activity 

 different elements of the chromatin in each case. 



B. THE NUCLEUS. 



The experiments we have so far considered have shown us 

 that while the nucleus is not qualitatively divided during 

 segmentation and subsequent development, the cytoplasm, far 

 from being isotropic, is heterogeneous, and its several parts 

 causally related to the differentiation of certain elementary 

 organs. 



It does not, however, follow for a moment that the nucleus 

 has no role to play in the process of differentiation, and there 

 are indeed very sound reasons for believing that it is a vehicle 

 of inheritable characters. 



1. For in the first place experiments on Protozoa have 

 proved that the nucleus, though not essential for irritability 

 and locomotion, nor even for the ingestion of food, is yet 

 necessary for the functions of metabolism and reproduction. 



An enucleate piece of an Amoeba can ingest and even 

 partially digest food, but the products of digestion cannot 

 be assimilated. Any portion of an Infusorian (that is not 

 too small), provided it contains a piece of the nucleus, can 

 replace those parts of the whole structure which it lacks, can 

 reproduce the original form. 



2. Secondly, the study of the maturation of the germ-cells 

 has shown that these elements, while unlike in every other 

 respect, are yet identical in the number and size of the 

 chromosomes of their nuclei (with the exception only of the 

 heterochromosomes or sex chromosomes in Insects and others). 

 Similar characters being inheritable from either parent, the 

 determinants of such characters have naturally been imagined 

 to reside in the nuclei, that is, in the chromosomes of the 



