Inheritance Materials of Germ-Cells 61 



that tlie most essential fact in fertilization is tlie union of 

 the nuclei of the male and female germ-cells. 



That cytoplasm is a physical basis of heredity is proved 

 by a great body of direct observational knowledge. That 

 the chromatin of chromosomes is a physical basis of heredity 

 is proved by much observational knowledge when this knowl- 

 edge is supplemented by reasoning involving the principles 

 of biological comparison and correlation. These two masses 

 of knowledge constitute, as already indicated, the founda- 

 tion of all legitimate reasoning about inheritance material. 

 Whether chromatin and cytoplasm are the only substances 

 which participate in the formation of hereditary structures 

 can not now be stated with certainty, though there are both 

 observational and general grounds for believing that they 

 are not. But into this question we need not enter in this 

 discussion. Nor is it necessary for our purpose to inquire 

 very particularly whether the conceptions of chromatin 

 and cytoplasm really imply just two substances or two great 

 classes of substances, though it is best to have in mind the 

 undoubted fact that the latter alternative is the true one. 

 Beyond a doubt "chromatin" and "cytoplasm" ought al- 

 ways to be understood to mean "kinds of chromatin" and 

 "kinds of cytoplasm." 



Function of Chromosomes in Heredity Acquired and 



Secondary 



The question which specially concerns us here is that of 

 what the relation is between chromatin and cytoplasm in 

 virtue of which they play the particular roles they are found 

 to play in producing hereditary structures. PiM-haps tlie 

 most important aspect of this general question is that wliich 

 the theory of phyletic evolution naturally brings up: does 

 the evidence in hand suggest any answer to the question 

 whether chromatin or cytoplasm is the more primitive and 



