68 PROBLEMS OF FERTILIZATION 



in this one particular they are demonstrably alike. 

 No wonder that this determination has furnished the 

 foundation for the most elaborate chromosome theories 

 from the time of Weismann until the present day, which 

 have been supported, changed, and rectified by the 

 most painstaking investigations of chromosome be- 

 havior in all stages of the cycle of the germ cells. 



This morphological and genetic equivalence is also 

 physiological in the sense that either germ nucleus is 

 adequate in itself for purposes of development. This 

 is proved for the egg nucleus by artificial partheno- 

 genesis, and for the sperm nucleus by those experiments 

 in which an enucleated fragment of an egg fertilized by 

 a single spermatozoon has been proved to develop 

 {merogony, see p. 162). 



Differences between the germ nuclei on the morpho- 

 logical side have been shown to occur between those 

 chromosomes which are concerned in sex determination 

 (Morrill, 1910; Mulsow, 1912; see Fig. 10); on the genetic 

 side differences exist, undeterminable morphologically, 

 which depend on the genetic history of the individual 

 and which are of an entirely similar order in both germ 

 nuclei. The foundation of all genetic theory of sexually 

 produced organisms thus rests upon the demonstrated 

 equivalence of the germ nuclei. 



2. Other constituents of the spermatozoon in the egg. — 

 In addition to the nucleus the spermatozoon usually 

 introduces certain cytoplasmic constituents into the 

 egg, but as contrasted with the nucleus there has been 

 the greatest difficulty in tracing the fate and determin- 

 ing the significance in fertilization of these elements, 

 which, moreover, vary greatly in different groups of 



