264 GENERAL ZOOLOGY 



of the animal developing from the egg is on the average as much 

 paternal as maternal. The chromosome theory of heredity finds 

 additional support in the fact that the biparental character of 

 the nuclei of the cells derived from the cleavage nucleus pro- 

 vides an equal opportunity for both maternal and paternal genes 

 to produce a biparental hereditary effect. 



The disjunction of homologous chromosomes that occurs in the 

 reduction division of gametogenesis may be interpreted in this 

 connection as the mechanism which is directly responsible for 

 the constancy in the total number of chromosomes, since without 

 the reduction there would be an ever increasing number pro- 

 duced at each fertilization. In cases where disjunction fails to 

 occur, or is incomplete, it can be shown that definite hereditary 

 effects are produced. Change in the ratio of autosomes to sex 

 chromosomes produces a definite effect in the characters of the 

 offspring, because of a change in the normal number and relations 

 of factors or genes. A further reason for assuming that the fac- 

 tors of hereditary characters are located in the chromosomes is 

 that such an assumption provides the only known explanation 

 of the results of animal and plant breeding, some of which will be 

 discussed presently. The science of genetics is in fact postulated 

 on this assumption. 



Cytoplasm in Heredity. — The chromosome theory of heredity 

 relegates the cytoplasm to a secondary role in development. 

 To students of heredity, the cytoplasm is rather superfluous 

 except as a source of nutrition or as a background for gene action. 

 The facts warrant the assumption that genes or factors account 

 for the inheritance of those characters that serve as material for 

 genetic study; but these characters, such as eye color, body 

 color, hair color, shape of legs and wings, etc., are largely super- 

 ficial characters and genetic studies as a rule fail to deal with 

 more fundamental characters such as those determining body 

 axes or body plan. The cytoplasm of the egg can be shown in 

 many cases to possess a certain amount of Organization that 

 foreshadows axial relationships in the later embryo, and this 

 must be taken into account in a complete explanation of heredity. 

 It will be recalled in this connection that in the frog's egg the 

 entrance of the sperm results in the formation of the gray crescent 

 and that in the majority of cases the future plane of bilateral 

 symmetry of the embryo is established by the entrance plane of 



