The Cellular Basis 131 



really independent in the sense that they can exist apart from one 

 another. They are to be thought of as analogous to chemical 

 radicals which are never independent but exist only in combina- 

 tion with other chemical elements in the form of molecules, and 

 yet preserve their identity in many different combinations. 



It is certain that Mendelian factors are not to be regarded as 

 gemmules or the germs of particular characters. There is not a 

 separate factor for every character, and factors are not "repre- 

 sentatives" or "carriers" of characters. They are the differen- 

 tial causes of particular characters just as in the compounds 

 H 2 SO 4 and K 2 SO 4 the hydrogen and potassium atoms are the 

 differential causes of the properties manifested by these two 

 substances. 



Location of Inheritance Units. If there are inheritance units, 

 such as determiners or genes, as practically all students of heredity 

 maintain, they must be contained in the germ cells, and it becomes 

 one of the fundamental problems of biology to find out where and 

 what these units are. There are many evidences that these genes 

 are located in the chromatin of the nucleus, that they are arranged 

 in a linear series when the chromatin takes the form of threads, or 

 chromosomes, preparatory to cell division, that in the division of 

 each chromosome every gene which it contains is also divided 

 and that daughter chromosomes and daughter genes are distrib- 

 uted equally to the daughter cells at every typical cell division 

 (Figs. 6, 7, 8). For nearly fifty years this complex process of 

 nuclear division, known as mitosis or karyokinesis, has been recog- 

 nized as a mechanism for the equal distribution of the chromo- 

 somes to the daughter cells, and for nearly that length of time it 

 has been suggested that the inheritance material or germplasm 

 was located in the chromosomes, but only within recent years has 

 critical experimental evidience been obtained that inheritance units 

 occupy definite positions in these chromosomes. With this ad- 

 vance in our knowledge, which we owe chiefly to Morgan and his 

 associates, it may be said that an important part, at least, of the 

 "mechanism of heredity" has been discovered. 



