440 HEREDITY AND DEVELOPMENT 



but the heterogeneity may spread into the individual chromosomes, 

 affecting the ids. 



Summary. — Put as simply as possible, the case is as follows. 



The independently heritable and variable qualities of an 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 fertilised 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 appar- 

 ently 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 cells 

 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. 



