24 Heredity and Eugenics 



more than a multitude of similar cells. In the second place, 

 there is cell differentiation, groups of cells becoming different, 

 so that the various tissues, with their special functions, are 

 developed. In the third place, tissues must be organized 

 together into the structures called organs. In the last 

 place, the organs must be combined in making that total 

 organization called the individual. It is this far-reaching 

 directive influence that is the most baffling fact in connection 

 with heredity. 



To obtain any impression of the supposed machinery of 

 heredity, it is necessary to know something of the structure 

 of a living cell. So far as material goes, such a cell is an 

 individualized mass of protoplasm. This protoplasm is 

 organized into a body known conveniently as the proto- 

 plast, which is the living body of the cell. In plants, the 

 protoplast usually constructs a cellulose wall about itself, 

 which has given rise to the impression that a cell is a walled 

 chamber containing protoplasm. In the plant cells which 

 have to do directly with heredity, however, namely the 

 reproductive cells, the cellulose wall is not formed, and 

 they are naked protoplasts. The protoplast is exceedingly 

 complex, as cytologists well know, and includes a variety 

 of organs. Conspicuous among these protoplasmic organs is 

 the nucleus, which is a more or less spherical body and usually 

 sharply limited from the rest of the protoplast, in which it 

 lies imbedded (Fig. i). The remainder of the protoplasmic 

 material enters into the structure of the cytoplasm, another 

 organ or region of the protoplast constantly associated 

 with the nucleus. Every living cell contains a nucleus and 

 cytoplasm, and in addition there rnay be other protoplasmic 

 organs (Fig. i), but the two mentioned are those that belong 

 to this discussion. 



