THE CHROMOSOME THEORY OF HEREDITY 301 



their issue with the sole exception of sex and associated char- 

 acters. 



Since the germ cells are actually a bridge between the genera- 

 tions and must therefore contain the potential equivalent of all 

 that is to appear in the new individual it is rcasona])le to expect 

 similarity of those which combine to the same extent that we 

 find similarity of the characters which they transmit. This simi- 

 larity of male and female germ cells we have but briefly considered. 



The development of sexual reproduction begins with the union 

 of obviously similar gametes. Differentiation such as that exhib- 

 ited by the germ cells of mammals and birds is an extreme develop- 

 ment. In the transition between the two stages it appears that 

 anything which remains constant is fundamental, and the chro- 

 matin of the nucleus alone fails to undergo marked change. The 

 highly specialized spermatozoon conveys little or no cytoplasm 

 into the ovum which it fertilizes; the ovum contributes much. 

 The nuclear material, especially the chromatin, is equivalent with 

 the exception of sex chromosomes. If cytoplasm played a large 

 part in the determination of the qualities of the new individual the 

 preponderance of maternal characters would seem to be inevitable. 



The Role of the Cytoplasm. The cytoplasm is apparently only 

 a plastic material for the expression of the heritage. Lillie 

 and Just make the significant statement that "the materials of 

 the cytoplasm are . . . being constantly consumed in the metab- 

 olism, and the process of renewal and increase of such materials 

 involves interaction of nucleus and cytoplasm; therefore the 

 purely maternal cytoplasm soon disappears, and is replaced by 

 cytoplasm formed under the influence of the biparental zygote 

 nucleus." There is certainly some increase, if not replacement, 

 of material in the chromosomes as development proceeds, but it 

 is evidently only to meet the demands of growth and multiplica- 

 tion, while cytoplasmic interchanges are at the root of the intricate 

 vital processes in the living Ijody. 



A most convincing experiment is that conducted by Boveri on 

 sea urchins. Enucleate cytoplasmic fragments from the eggs of 

 one species were fertilized by the sperms of another species. 

 These fragments developed sufficientl}^ to show definite charac- 

 ters, and the characters produced were those of the species from 

 which the sperms were derived. The wholly cytoplasmic maternal 

 contribution was apparently without (effect. 



