BALANCE AND UNBALANCE 315 



geneous observations into a consistent scheme we can see three 

 principles connecting the differential staining capacity with other 

 differential properties of the chromosomes. First, there is the 

 inertness of the heterochromatic regions of the sex and other 

 chromosomes in Drosophila, Chorthtppus and Zea. Secondly, there 

 is the reversal of staining properties in Rattus, Chorthtppus and 

 Drosophila at different stages of the nuclear cycle. Thirdly, there is 

 the attraction or lapse of repulsion of heterochromatic chromo- 

 somes at one stage in Rattus, Chorthtppus, Zea and elsewhere. 



The first of these shows the working of a genetic property in the 

 particles or genes concerned, and agrees with the particular observa- 

 tions of the last type of differential behaviour described above. 

 The second shows that the behaviour depends essentially on a 

 difference from the substrate, such, that if the usually over-con- 

 densed parts of the chromosomes have the difference the under- 

 condensed parts have not, and vice versa. What this difference is 

 can be seen from the third principle : the absence of repulsion 

 indicates a lower surface-charge and therefore probably a difference in 

 iso-electric point of the chromosome materials. This agrees with the 

 constant material basis in the genes arrived at earlier. But further 

 it seems very probable that the staining capacity of the chromosomes 

 with ordinary treatment should be related to the surface charge 

 on them (although, as we find, not with the Feulgen reaction), 

 since the deposition of the common nuclear dyes is a surface 

 phenomenon. 



We may conclude, therefore, provisionally, that the differential 

 staining behaviour of chromosomes is conditioned most immediately 

 by differential surface charge which in turn depends on the difference 

 between the local iso-electric point and the pH of the medium, and 

 also (in the case of proximal condensation) on more remote con- 

 ditions such as the proximity of the heavily charged centromere 

 (cf. Ch. XII). 



3. BALANCE 

 Boveri showed from experiments on doubly fertilised sea-urchin 

 eggs that all the chromosomes making up the complement of a 

 gamete must be present for the normal development of the zygote. 



