Physical Basis of Heredity and Evolution 



27 



in the same sense that protoplasts are protoplasm indi- 

 vidually organized, and they are thought to retain their 

 indi\idual identity through all the apparent fusions into 

 bands and network. It is the chromosome, therefore, 

 consisting of the material chromatin, that is regarded as the 

 organized carrier of transmissible characters; hence the 

 behavior of the chromosomes in cell division becomes a 



e f g h 



Fig. 2. — Diagram of stages in nuclear division. — After Lock 



matter of first importance in considering the machinery of 

 heredity. 



One of the important facts to note is the definite number 

 of chromosomes. Each kind of plant and animal has its 

 o\vn number. For example, in certain plants the number 

 is 6; in others it may be considerably m.ore than 100; and 

 of course the intermediate numbers are well represented. 

 Differences in the number of chromosomes may occur 



