Cell Division and Heredity 69 



continuous and comes to an abrupt end with the 

 death of the individual. It is the germ cells and 

 not the somatic cells which contain the pattern 

 that is transmitted from generation to generation 

 because the progenitors of the germ cells do not 

 undergo any such irreversible alterations but only 

 pass through a series of cell divisions and even- 

 tually produce the next generation of germ cells. 

 There are two different kinds of inheritance. 

 The major anatomical characteristics by which 

 the different species of plants and animals are 

 distinguished from one another reappear from 

 generation to generation and remain unaltered 

 until there is some permanent change in the char- 

 acteristics of the species. When such a change 

 does take place it is always irreversible and not 

 governed by Mendel's laws, from which it ap- 

 pears that the corresponding alterations in the 

 spirazine pattern of the germ cells must also be 

 irreversible. Such alterations may consist of 

 changes in the total number of spirazines or in 

 their connections with one another through the 

 alpha carbon atoms. On the other hand there are 

 certain minor characteristics, such as the colors 

 and textures of the various body-tissues, which 

 may change freely from generation to generation 

 in accordance with MendePs laws without involv- 

 ing any permanent change in the nature of the 



