CHAPTER LIV. 



r DETERMINATION OF SEX GROWTH AND 

 SENESCENCE. 



Heredity. The development of the fertilized ovum offers two 

 general phenomena for consideration: First, the mere fact of mul- 

 tiplication by which an infinite number of cells are produced by 

 successive cell-divisions; second, the fact that these cells become 

 differentiated in structure in an orderly and determinate way so as 

 to form an organism of definite structure like those which gave 

 origin to the ovum and the spermatozoon. In other words, the 

 fertilized ovum possesses a property which, for want of a better 

 term, we may designate as a form-building power. It is this last 

 peculiarity which is included under the term heredity. The ovum 

 develops true to its species, or, indeed, more or less strictly in accord- 

 ance with the peculiarities of structure characteristic of its parents. 

 The object of a complete theory of heredity is to ascertain the me- 

 chanical causes that is, the physicochemical properties resi- 

 dent in the fertilized ovum which impel it to follow in each case a 

 definite line of development. The discussions upon this point have 

 centered around two fundamentally different conceptions designated 

 as evolution and epigenesis. 



Evolution and Epigenesis. The earlier embryologists found a 

 superficial explanation of this problem in the view that in the germ 

 cells there exists a miniature animal already preformed, and that 

 its development under the influence of fertilization consists in a 

 process of growth by means of which the minute organism is 

 unfolded, as it were. The process of development is a process of 

 evolution of a pre-existing structure. Inasmuch as countless in- 

 dividuals develop in successive generations, it was assumed also 

 that in the germ cell there are included countless miniature organ- 

 isms, one incased, as it were, in the other. Some of the embry- 

 ologists of that period conceived that the undeveloped embryos are 

 contained in the ovum, the ovists, while others believed that 

 they are present in the spermatozoon, the animalculists. Other 

 embryologists pointed out that the fertilized egg shows no indication 

 of a preformed structure, and therefore concluded that development 

 starts from an essentially structureless cell and consists in the 

 successive formation and addition of new parts which do not pre- 

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