128 Heredity and Environment 



undifferentiated ; both egg and sperm are differentiated, the for- 

 mer for receiving the sperm and for the nourishment of the em- 

 bryo, the latter for locomotion and for penetration into the egg. 

 But while the differentiations of tissue cells are usually irreversi- 

 ble, so that they do not again become germinal cells, the differen- 

 tiations of the sex cells are reversible, so that these cells, after 

 their union, again become germinal cells. The ovum loses its 

 power to form yolk and during the early development it gradually 

 loses all the yolk which it had stored up ; the spermatozoon loses 

 its highly differentiated tail or locomotor apparatus and its small 

 compact nucleus absorbs substance from the cytoplasm of the 

 egg and becomes a large germinal nucleus. 



Chromatin is Germplasm, Cytoplasm is Somatoplasm. In many 

 theories of heredity it is assumed that there is a specific "inheri- 

 tance material," distinct from the general protoplasm, the func- 

 tion of which is the "transmission" of hereditary properties from 

 generation to generation, and the chief characteristics of which 

 are independence of the general protoplasm, continuity from 

 generation to generation and extreme stability in organization. 

 This is the idioplasm of Nageli, the germplasm 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 germplasm, 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- 



