WEISIVL^NN'S THEORY OF HEREDITY 49 



Weismann's Mechanism of Heredity 



Weismann had this advantage over Darwin; in his time 

 knowledge of the structure of the germ-cells had considerably 

 increased over what it was when Darwin conceived the hy- 

 pothesis of pangenesis. 



Weismann identified his " determiners " with certain con- 

 spicuous structures of the germ-cell called chromosomes 

 (unknown in Darwin's time), and supposed that the nature 

 of these determines and controls the nature and activity of 

 the cell containing them. 



It is the theoretical importance which Weismann and 

 others have assigned to these structures that has given them 

 their great prominence in the study and description of cell 

 phenomena in the last thirty years. In reality the chromo- 

 somes make up a part only of the germ-cell and we have no 

 certain knowledge that they form the more important part. 

 Nevertheless a majority of biologists, probably, at the present 

 time believe with Weismann that heredity is due to material 

 substances or determiners which are located in the chromo- 

 somes. The principal reasons for so thinking are: 



1. The conspicuousness of the chromosomes at the time 

 of cell division and the very exact manner in which as a rule 

 each of them divides into two equal parts, which pass into 

 different cell-products. 



2. The constancy of the number of the chromosomes in 

 the same species of animal or plant. The number is different 

 in different species but within the same species it is very con- 

 stant. The only known exceptions to this rule are such as 

 may be cited in support of the general idea that chromosomes 

 are determiners of heredity. 



(a) The two sexes within the same species frequently differ 

 as regards the number of chromosomes in their gemi-cells. 

 When this is the case the male has the smaller number of 

 chromosomes, and it is assumed that the chromosome or 

 chromosomes which the male lacks determine femaleness. 



(6) It has been shown in the case of the evening primroses 

 (Oenothera) that a particular heritable type of variation 



