CHROMOSOME CONSTANTS 53 



The developmental conditions of rapid mitosis reduce spiralisation 

 below the constant value (as in Aggregata), while in pre-meiotic 

 mitoses of animals it may be increased (as in Chorthippus, Fig. 11). 



2. GENOTYPIC CONTROL 

 Considering these constants, we can see that there are some that 

 can be taken as expressing the permanence of the thread which goes 

 to make the metaphase chromosome. Thus the potentiality for a 

 constriction, centromere or nucleolar, and the relative length of the 

 chromosomes must be directly determined by the permanent 

 character of the individual chromosomes. This character is, how- 



FiG. II. — Metaphase in early and late spermatogonial mitoses of 

 Chorthippus parallelns showing different degrees of spiralisation 

 (2w = i6 + X). X understained, c/. Chs. VIII and IX. x 2500 

 (D.. 1936^.) 



ever, consistent with variation in the appearance of the chromosomes 

 in other respects. The constant diameter of the chromosomes 

 making up one complement especially suggests that this diameter 

 has nothing to do with the permanent structure of the individual 

 chromosomes, but is a racial, i.e., a genetic character, and therefore 

 subject to variation through genetic change [cf. Ch. I). 



This means that the genetic properties of the organism, which 

 are controlled by certain permanent materials (the genes) in the 

 chromosome thread, themselves control, within certain limits, the 

 form that this thread takes up at metaphase of mitosis. It is of 

 the utmost importance in interpreting observations of chromosomes 

 in terms of genetics to know what these limits are. We must 



