The Cellular Basis 131 



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 forty 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 evidence 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. 



It must be said however that there are biologists who still re- 

 fuse to believe that heredity is associated with any particular cell 

 substance, while many others who would grant this are not yet 

 ready to admit that there are particular units or genes which are 

 concerned in the production of particular characters. However 

 anyone who will examine at first hand the evidences in favor of 

 this cannot fail to be impressed with its importance, and no one 

 has proposed any other hypothesis that is at all satisfactory. But 

 whether we assume the existence of these units or not we know 

 that the germ cells are exceedingly complex, that they contain 

 many visible units such as chromosomes, chromomeres, plasto- 

 somes and microsomes, and that with every great improvement 

 in the microscope and in microscopical technique other structures 

 are made visible which were invisible before, and whether the par- 

 ticular hypothetical units just named are invisible or not seems to 

 be a matter of no great importance, seeing that, so far as the 

 analysis of the microscope is able to go, there are in all proto- 

 plasm differentiated units which are combined into a system; in 

 short, there is organization. 



