DIFFERENTIATION, HEREDITY, SEX 261 



Heredity is the fact of resemblance between offspring and 

 parents, not the resemblance of adult stages alone, but the 

 likeness at all corresponding ages. That the individual ova of 

 Asterias vulgaris are alike, that the cleavage processes, blastulas, 

 gastrulas, larvae, and adolescent stages of all the individuals 

 of this species are essentially alike, in structure and in behavior 

 all this is similarly the fact of heredity. A specific kind 

 of protoplasm is never, whatever its form, anything other than 

 that specific kind. 



In other words, the interest of the embryologist in the prob- 

 lems as to why the ovum develops as it does, passes from one 

 condition to another as it does, and finally produces the kind 

 of adult that it does, is essentially an interest in the problem 

 of heredity the problem of organismal specificity. This is 

 the central point of embryological study. Consequently we 

 are fully justified in considering in this place, these general 

 problems of the relation of the facts of cytology and embryology 

 to the facts of parental and specific likeness of organisms. 

 Failure to do so would mean the omission of the most vital 

 topic around which much, perhaps it would not be going too 

 far to say most, of recent embryological investigation has 

 centered, and upon which it is to-day focussed. 



The answers which the facts of embryology have to offer to 

 these fundamental questions are still rather vague and uncer- 

 tain. Most of them are stated in the potential mood and must 

 still be framed as hypotheses. But although the facts here may 

 be much clearer than their significance, we must attempt a state- 

 ment of both ; and it goes almost without saying that our treat- 

 ment of both must be as brief and as elementary as possible; 

 this is not the place for extended consideration of hypothetical 

 views, however great the importance of the central ideas. 



We may address ourselves, therefore, to a survey and brief 

 analysis of the answers which have been given to the question 

 why the organism develops in the way it does. First, let us 

 recall the idea, stated briefly in the introductory chapter, that 

 development is a form of behavior a series of reactions. In 

 any organic reaction the two factors of external and internal 



