CHAPTER XIII 



HEREDITY 



A fertilized frog's egg develops into a frog, rather than into 

 a salamander or some other kind of animal, because it is endowed 

 with certain inherited qualities, which under proper environ- 

 mental conditions bring about development. Development is 

 accompanied or caused by an interaction of internal factors, 

 present in the egg, with external factors present in the environ- 

 ment. A certain sort of environment is required for the develop- 

 ment of the frog's egg, but since the eggs of other species may 

 develop in the same pool along with those of the frog, the factors 

 determining the difference in results in each case must be internal 

 rather than external. There is specificity in different kinds of 

 eggs just as there is specificity in the animals developing from 

 them. Each kind of egg is characterized by its own particular 

 kind of developmental potencies which, however, are realized 

 only when provided with the proper environment. The same 

 environment may evoke the potencies of different kinds of eggs; 

 but the same kinds of eggs require the same kind of environment 

 for their development. 



In the intrauterine development of mammals there is a 

 similar relation between the egg and environment, the environ- 

 ment in such cases being the uterine tissue surrounding the egg. 

 The uterus is the means through which the metabolic needs of 

 the developing egg, and later the embryo, are met; and thus 

 provides an environment in which the potentialities of the 

 hereditary factors present in the egg can be expressed in the form 

 of developmental activity. Theoretically, therefore, a mam- 

 malian egg should develop equally well in the proper sort of 

 artificial culture medium outside the body; and, as a matter of 

 fact, this has been accomplished with rabbit ova through early 

 cleavage stages. As in the frog, the mammalian offspring 

 inherits from its parents only those qualities that are repre- 

 sented by factors of some sort in the fertilized egg. 



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