Mechanism of Mendelian Heredity 251 



purposeful reactions based on these tropisms that a 

 factor for the formation of the photosensitive substance 

 should exist. 1 



5. Another point should be emphasized, namely 

 that for Mendelian heredity it is immaterial whether 

 the character is introduced by the spermatozoon or 

 by the egg. This fact which Mendel himself already 

 recognized is in full harmony with the conclusion that 

 the chromosomes and not the cytoplasm are the bearers 

 of Mendelian heredity, since only in respect to the 

 chromosome constitution are egg and sperm alike, 

 while they differ enormously in regard to the mass of 

 protoplasm they carry. We can, therefore, be tolerably 

 sure that wherever we deal' with a hereditary factor 

 which is determined by the egg alone the cytoplasm 

 of the latter is partly or exclusively responsible for the 

 result. 



We have already mentioned the fact that the rate of 

 segmentation of the egg is such a character. Yet 

 this character is as definite as any Mendelian character, 

 and it would be as easy to discriminate two species 

 of eggs by the time required from insemination to the 

 beginning of cell division as it would be by any Men- 

 delian character of their parents. 



The application of our modern knowledge of heredity 

 to human affairs has been discussed in a very original 



1 Loeb, J., "Egg Structure and the Heredity of Instincts," The Monist, 

 1897, vii., 481. 



