26 Heredity and Eugenics 



the two new cells. In ordinary cell division, the imme- 

 diate transmission seems to be a similar structure, for the 

 new cells resemble the old one in all recognizable features. 

 In the differentiation of cells, however, certain cell divisions 

 involve the transmission of unlike characters. 



This relation of the nucleus to cell division, and of cell 

 division to heredity, has focused the attention of cytolo- 

 gists upon the structure and behavior of the nucleus. No 

 structure of plants and animals has received such detailed 

 and persistent investigation as has the nucleus, and much 

 of the advance in technique associated with the use of the 

 microscope has been stimulated by the necessity of learning 

 more about the nucleus. 



If the nucleus is the conspicuous structure associated 

 with cell division, the suggestion is natural that it is the 

 material structure associated with heredity. But the 

 nucleus is a complex, and most conspicuous in its structure 

 is a substance called chromatin. In the ordinary nucleus 

 it appears as a network of denser material, which has 

 received its name from the fact that it takes stains easily, 

 being the most stainable substance in the nucleus (Fig. 

 2, a). If there is any definite material that deserves to be 

 called the physical basis of heredity, it is probably chro- 

 matin, which of course is a protoplasmic substance. This 

 belief is largely based upon the behavior of chromatin 

 during cell division. 



In preparation for division, the chromatin network 

 becomes an e\ddent continuous band, which resembles a 

 tangled skein (Fig. 2, &). This band shortens and corre- 

 spondingly thickens, and finally breaks up into a definite 

 number of units, called chromosomes (Fig. 2, c). These 

 chromosomes are thus chromatin individually organized. 



