SPECIAL PROBLEMS OF MITOSIS 43 



The identification of sex-chromosomes is naturally 

 difficult when the X and Y are cytologically indis- 

 tinguishable. Where they are visibly different it is 

 usually possible to identify them by careful compari- 

 son between the somatic chromosome sets of the two 

 sexes. In the more highly evolved types of sex- 

 determining mechanism the sex-chromosomes, or at 

 any rate parts of them, are often clearly recognizable 

 by the fact that they contract during prophase at a 

 different rate to the autosomes (usually slower). Thus 

 in the spermatogonial mitoses of the Acrididae and 

 Gryllidae (Grasshoppers and Crickets) the greater 

 part of the X has only reached the mid-prophase 

 degree of contraction by the time the autosomes are 

 in metaphase.^^i' ^^^ This process of differential 

 condensation is known as heteropycnosis . In the 

 above case we can speak of negative heteropycnosis ; 

 the opposite condition of positive heteropycnosis 

 (where the sex-chromosome reaches the metaphase 

 degree of contraction when the autosomes are still in 

 early prophase) is often found in the first meiotic 

 division (see Chap. 5).'^' ^^ 



Heteropycnosis is not confined to sex-chromosomes ; 

 thus in many species of grasshoppers there is one 

 autosome which shows strong positive heteropycnosis 

 in about two -thirds of its length at the first meiotic 

 division. ^^ Moreover in Drosophila melanogaster all 

 the inert regions show heteropycnosis at the somatic 

 divisions. There seems to be some sort of connexion 

 between inertness and heteropycnosis, but it is not 

 possible as yet to state definitely that all heteropyc- 

 notic regions are more or less inert and vice versa. 



In a number of groups of organisms sex-determina- 

 tion does not depend on a pair of sex-chromosomes, 

 but on whether the eggs are fertilized or develop 

 parthenogenetically. Thus in these cases the males 

 are haploid , the females diploid . This is the case in the 

 Hymenoptera, the Mites (some species at any rate) and 

 in some Scale Insects (Coccidae). In the Hymenoptera 



