THE CHROMOSOME THEORY OF HEREDITY 303 



mutually dependent; cytoplasm alone can give expression to the 

 chromosomes and the chromosomes alone can activate the cyto- 

 plasm. 



Mitochondria. These are bodices included in the cytoplasm, to 

 which functions in heredity have been attributed. A paragraph 

 from Lillie and Just will serve as an ample refutation of this pro- 

 posal: "Whatever may be the function of the mitochondria in cell 

 physiology, it must be admitted that the study of fertilization 

 has shown no reason for the assumption that their introduction 

 into the egg by the sperm in certain species is concerned in the 

 transmission of paternal characteristics. The variable quantity 

 in different cases and the distribution to single blastomeres in 

 certain cases exclude the hypothesis that they have any specific 

 paternal hereditary effect. There is no reason to deny that sperm 

 mitochondria function in the egg when present, but if so it is prob- 

 able that they are not differentiated in their chemical composition 

 or genetic behaviour from the mitochondria of the egg itself." 



Mendelism and the Chromosomes. There is no stronger evi- 

 dence for the chromosome theory of heredity than the close resem- 

 blance between the behaviour of Mendclian unit characters and 

 that of the chromosomes. The points of resemblance may be 

 summarized as follows: 



1. Unit characters are duplicated in the organism, as is shown 

 by the behaviour of allelomorphic units when they are combined 

 in a hybrid. The monohybrid ratio indicates that there are two 

 and no more than two of these units present, since it expresses 

 almost exactly the mathematical possibilities of such a number. 

 The same duplication prevails in the chromosomes of the body, 

 where the diploid condition is normal. 



2. Segregation of the determiners for unit characters occurs 

 during reproduction. The reappearance of recessive characters 

 in the Fo generation after their complete concealment in the 

 hybrid parents is evidence of this segregation. This is a close 

 parallel of the separation of synaptic mates during the reduction 

 division in gametogenesis. 



3. The characters derived from hybrid parents are a recombina- 

 tion of characters represented in some way in both parents. In 

 the formation of gametes and their union to form ncnv individuals 

 the possil^ility of similar rearrangement of parental chromosomes 

 is apparent. 



