HEREDITY AND ENVIRONMENT IN DEVELOPMENT 439 



If it is found that adult characteristics appear in a certain manner, 

 the probable arrangement of whatever it is in the zygote that 

 represents these characteristics can be inferred. 



An amazing result of the recent experimental work upon the 

 heredity of adult characteristics is that the knowledge thus gained 

 enables us to picture, without seeing, certain characteristics of 

 the organization of the germ cells, much as the chemist pictures the 

 organization of molecules. There is, however, one respect in 

 which the biologist, who seeks to understand the organization of 

 the germ cells, has an advantage over the chemist, who postulates 

 the structure of invisible molecules. There exists within the 

 nucleus of ovum and spermatozoon, as in all other cells, a visible 

 substance, chromatin, that appears at the time of cell division in 

 the form of chromosomes (cf. Fig. 76, p. 138), which are constant, in 

 number and appearance, for any given species. The behavior of 

 these chromosomes, as seen by the microscope, is so specifically 

 related to the end results of heredity as virtually to identify them 

 with the mechanism of inheritance for certain adult qualities, and 

 hence to suggest the probable organization of the germinal sub- 

 stance. It is thus possil)le to attack this aspect of the problem of 

 development at its two extremes and to correlate the attacks so 

 that each supplements the other. 



Heredity and Environment in Development. — Although the 

 discussion of heredity is reserved for a later chapter, we may 

 here consider the relative importance of hereditary and environ- 

 mental factors in the development of the individual {cf. p. 484). 

 The physical basis of heredity is found in the germ cells and in the 

 processes of development which have been described in the fore- 

 going chapters. We have, therefore, become somewhat acquainted 

 with the mechanism of heredity as it operates during development, 

 if not with the organization of the germ cells and inheritance as 

 described in the chapter on Genetics. The development of an 

 individual from the germ cells is a result of the internal factor of 

 heredity and the external factor of environment, or of "nature" 

 and "nurture" as they are often called. On the one hand, the 

 zygote that is formed by the union of ovum and spermatozoon 

 possesses certain potentialities; on the other, it is subjected to 

 environmental influences at every stage of its development. The 

 famous experiment by Castle (Fig. 229) in which the ovary of a 

 black guinea pig was transplanted into a white female, which was 



