32 



THE GERM -PLASM 



arrangement, number, and nature of the cells which compose 

 it. The influence therefore which the minute mass of paternal 

 chromatin in the nucleus of the fertilised egg-cell exerts on 

 the course of development, can only be such as to regulate the 

 nature and the rate of multiplication of the cells in the body of 

 the offspring in such a manner as to cause them to resemble 

 the cells of the paternal body. The chromatin is therefore iji a 

 condition to impress the specific character oji the cell in the 

 nucleus of ivhich it is contaitied. 



As the thousands of cells which constitute an organism pos- 

 sess very different properties, the chromatin which controls them 

 cannot be ii7iiform ; it must be different in each kind of cell. 



The chromatin, moreover, cannot becoine different in the cells 

 of the fully formed organism ; the differences in the chromatin 

 controlling the cells must begin with the development of the egg- 

 cell, and must increase as development proceeds ; for otherwise 

 the different products of the division of the ovum could not give 

 rise to entirely different hereditary tendencies. This is, how- 

 ever, the case. Even the two first daughter-cells which result 

 from the division of the egg-cell give rise in many animals to 

 totally different parts. One of them, by continued cell division, 

 forms the outer germinal layer, and eventually all the organs 

 which arise from it — e.g.., the epidermis, central nervous system, 

 and sensory cells ; the other gives rise to tlie inner germinal 

 layer and the organs derived from it, — the alimentary system, 

 certain glands, &c. The conclusion is inevitable that the 

 chromatin determining these hereditary tendencies is different in 

 the daughter-cells. 



This holds good in all subsequent stages of ontogeny ; the 

 difference between the developmental tendencies of the cells 

 resulting from the division of the ovum is in exact proportion to 

 that between the chromatin substance of their nuclei. Ontogeny., 

 or the development of the individual, depends therefore on a 

 series of gradual qualitative chaiiges in the nuclear substance of 

 the egg-cell. 



The fundamental principle of the view which has just been 

 briefly sketched was put forward by me some years ago, and I 

 then made use of the term idioplasm to represent the substance 

 which is contained in the chromatin bodies of the nucleus, and 

 which determines the nature of the whole cell. Oscar Hertwig 

 also independently adopted this term, which had first been intro- 



