128 Heredity and Environment 



This is the idioplasm of Nageli, the germ-plasm of Weismann. 

 Such a substance is no mere fiction or logical abstraction, as 

 many writers have affirmed, for there is in the nucleus of every 

 cell a substance which fulfills all of these conditions, namely, 

 the chromatin. It is relatively independent of the surrounding 

 cytoplasm, it is self -propagating and consequently continuous 

 from cell to cell, and from generation to generation and it is 

 relatively stable in organization so that it is but little influenced 

 by environmental conditions. There are many important rea- 

 sons for believing that the chromatin is the germ-plasm, or at 

 least that it contains the inheritance units, as we shall see later. 

 It is present not only in germ-cells but in every cell of the organ- 

 ism, though in highly differentiated tissue cells it may undergo 

 certain secondary modifications. On the other hand the cyto- 

 plasm surrounding the nucleus, undergoes many marked differ- 

 entiations in the course of development and it constitutes in the 

 main the body plasm or somatoplasm. Germplasm and somato- 

 plasm are not, therefore, vague generalizations, but they are defin- 

 ite cell substances which may be seen under the microscope. 



5. The Units of Living Matter. — The entire cell, nucleus and 

 cytoplasm, is the smallest unit of living matter which is capable 

 of independent existence. Neither the nucleus nor the cytoplasm 

 can for long live independently of each other, but the entire cell 

 can perform all the fundamental vital processes. It transforms 

 food into its own living material, it grows and divides, it is capa- 

 ble of responding to many kinds of stimuli. But while the parts of 

 a cell are not capable of independent existence they may be dif- 

 ferentiated to perform different functions. 



Panmerism. — Not only is the cell as a whole capable of assimi- 

 lation, growth and division, but every visible part of the cell has 

 this power. The nucleus builds foreign substances into its own 

 substance, and after it has grown to a certain size it divides into 

 two ; the cytoplasm does the same, and this process of assimila- 

 tion, growth and division occurs in many parts of the nucleus 



