ORGANIC DIFFERENTIATION 85 



medium and thus be reproduced exactly, each in its relative 

 situation. The second hypothesis is more plausible, but it 

 assumes that the reproductive cells succeed in evolving only 

 when all the characters of the organism to which they belong 

 are developed. It is true (except in rare cases) that it is at 

 maturity that the reproductive cells of animals become ready 

 to develop — and any exceptions to this rule are either apparent 

 only or explicable by the phenomena of tachygenesis. Among 

 the structural cells, the most active agents of transformation 

 are those which direct the phenomena of nutrition and thus 

 play the principal part in building up form ; these belong to the 

 category of soluble ferments. If the very extensive action of 

 these ferments is reversible, and they prove capable, under 

 certain circumstances, of rebuilding what they have destroyed, 

 and if the reproductive cells contain some ferments of this 

 nature, they ought to be capable of building up again, according 

 as the division of the blastomeres increases, the series of 

 substances composing the elements through which they had 

 originally passed in order to attain the egg-stage. Accordingly 

 as the new elements appear, they will assume the form and the 

 properties of the various parental elements. 



Is there anything in the structure of the reproductive cells 

 that would lend support to this hypothesis ? In order to answer 

 this question we must now show exactly how the structure 

 of body-building cells and reproductive cells is common to both 

 at the beginning of their existence. 



Every structural cell, as we know, is made up of a mass of 

 protoplasm (a mixture of various substances) protected by or 

 innocent of a membrane ; in the centre of this cell is a delicate 

 vesicle isolated from other substances, which constitutes 

 the nucleus. The most remarkable substance within the 

 nucleus itself is that which has received the name of chromatin x 

 because it possesses a special ability to fix colouring matter 

 and particularly ammoniacal carmine. During the period of 

 bipartition and multiplication of cells, the chromatin 

 contained in the nucleus normally arranged in the form 

 of a network, shrinks into a sinuous ribbon, the loops 

 of which, constant in number — with rare exceptions 

 — for all the cells of organisms of the same species, soon 



1 xpcu^a, colour. 



