342 The Unity of the OrgamRm 



evidence of this class, tliat many students devoting them- 

 selves exclusively to genetics seem not to realize that they 

 are dealing with such evidence. Reference is here made to 

 the truly brilliant researches of the last years proving be- 

 yond a doubt that in the sexual mode of propagation of 

 many plants and animals some sort of interdependence exists 

 between the attributes of tlie developed organism and the 

 chromosomes of the germ-cells. 



Since it is taken as proved that such an interdependence 

 exists we are not required to examine criticall}'^ the evidence 

 itself. Rather are we to inquire concerning the nature and 

 meaning of that interdependence. 



The Chromosomes of Germ-Cells in Fertilization 



The field is one of magnitude and complexity, and we can 

 touch only its prominent landmarks. The earliest known 

 class of facts, as well as one of the weightiest, favorable to 

 the theory concerns the part played by the chromosomes 

 of tlie male and female germ-cells in fertilization, the struc- 

 ture and behavior of the male cells being especially impor- 

 tant. It is now an established fact that the head of the 

 male reproductive cell, the spermatozoan, consists mainly 

 of the transformed nucleus of the spermatid, that is, the 

 cell from which the sperm is immediately derived, and that by 

 far the larger mass of the liead comes from the chromosomes. 

 In fact, so demonstrably large a portion is thus derived that 

 the statement is made over and over again in recent dis- 

 cussions that the sperm head is "practically entirely" of 

 chromatin. And since this })art of the spermatozoan is 

 proved to be the predominant element in fertilization, and 

 since the offspring inherit from the father no less than from 

 the mother, the inference has been widely drawn and firmly 

 held that the chromosomes nuist be mainly, if not exclusively, 

 the "heredltar^^ substance". It is, however, generally admit- 



