Physical Basis of Heredity and Evolulion 



27 



in the same sense that protoplasts are protoplasm indi- 

 vidually organized, and they are thought to retain their 

 individual 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 

 own number. For example, in certain plants the number 

 is 6; in others it may be considerably more than 100; and 

 of course the intermediate numbers are well represented. 

 Differences in the number of chromosomes mav occur 



