444 HEREDITY AND DEVELOPMENT 



organism are represented in the young germ-cell by a number 

 of material elements (determinants). 



As the young egg-cell ripens it divides in such a way that its 

 determinants are reduced in number by one-half. Not that it 

 need lose any particular kind of determinant, corresponding let 

 us say to the colour of the eye or the colour of the hair, for each 

 kind of determinant is represented in multiplicate. It loses 

 one-half of its sets of determinants. The same happens with 

 the ripening sperm-cell. 



When the mature egg-cell is fertiUsed by the mature sperm- 

 cell, the number of sets of determinants is once more raised to 

 what it was in the young cells before maturation. But though 

 the number of sets is the same as before, the collocation of the 

 sets is not the same. At any rate, it need not be the same ; 

 for there is an apparently random reduction. 



The character of the offspring depends upon the adjustments 

 arrived at among the different sets of determinants of maternal 

 and paternal origin. 



Hypothesis of Development. — Postulating an equipment of 

 primary constituents or determinants within the germ-plasm, 

 Weismann proceeded to elaborate a hypothesis as to the manner 

 in which these determinants determine the cells or cell-groups 

 to which they correspond. 



The fertilised egg-cell divides and redivides, and at first 

 the resulting cells (blastomeres) of the embryo are often equiva- 

 lent to one another. This is demonstrable experimentally, 

 for if the first four cells of the lancelet's ovum, for instance, 

 be shaken apart, each goes on developing on its own account 

 and forms a complete larva. In other cases, the resulting ceUs 

 are heterogeneous from the first division onwards ; and, in any 

 case, they soon become heterogeneous — that is to say, they form 

 certain parts of the embryo, and these only. In other words, 

 there must be a distribution of determinants in the course of 

 segmentation. 



