n6 SCIENCE PROGRESS. 



only, for in that case each blastomere would go through a 

 normal development, and give rise to a complete embryo ; it 

 must also affect the quality of the ovum, and separate off 

 that which belongs to the right half of the future organism 

 from that which belongs to the left half. There must be in 

 consequence a particular architecture in the protoplasm of 

 the ovum, an organisation of a definite kind, and of such a 

 nature that the characters of the future organism are sever- 

 ally represented in the separate structural elements of the 

 ovum, and that these become segregated in the course of 

 cell multiplication. 



It is really of no consequence to the argument whether 

 the determining structural complexity is supposed to reside 

 in the cell body, or in the nucleus ; it is the case that nume- 

 rous considerations have led to the belief that it resides in 

 the chromatin of the nucleus, but if it were otherwise, the 

 main proposition would be unaffected. 



The final conclusion is that, homogeneous and simple 

 as it may appear to our vision, the ovum is an exceedingly 

 complex and an organised body, and when this conception 

 is joined with that of particular vitales, which go to make up 

 the cell and to stamp it with its special characters, the further 

 conclusion is irresistible, that there are in the germ plasm 

 primordial form elements, of which each has an allotted 

 course by which it proceeds to its position in the adult 

 organism. The final result is attained when only one kind 

 of primordial element is contained in each cell, viz., that 

 which has to determine it. Weismann extends this idea by 

 supposing that the primordial elements are formed into 

 groups, and that the groups, which he calls determinants, 

 are segregated in the course of ontogeny until every cell, 

 with some few exceptions, contains and is controlled by only 

 one kind of determinant. 



The essence of the evolution theory consists in the 

 view that the primordial particles contained in the germ 

 are of as many different kinds as there are different kinds 

 of cells in the adult organism ; that the course followed by 

 each is predetermined by the position which the cell or cell 

 group which it controls will have in the adult ; and that in 



