138 CYTOLOGY 



CHAP. 



some individuals this chromomere is present in both members of the 

 pair, in some it is absent from both members, and in others it is present 

 in one chromosome but absent from its homologue. The same combina- 

 tion is of course constant for all the nuclei of a given individual. The 

 existence of the three possible combinations indicates promiscuous 

 syngamy between gametes which possess and those which do not possess 

 the chromomere in question. 



Inequalities, or other visible differences, between the two members of 

 a homologous pair have also been described in Orthoptera by Carothers 

 {Brachystola, 1913; Trimerotropis and Circotettix, 1917) and by Robertson 

 (Tettigidea and Acridium, 1915)- 



In Trimerotropis (one of the grasshoppers) the metaphase chromosomes 

 are either rod-shaped, or bent into V's (with equal or unequal arms). 

 These shapes are not transitory forms impressed on the chromosomes by 

 temporary forces acting in mitosis, but they mark the different methods 

 of attachment of the spindle fibres to the chromosomes, and are constant 



in all the nuclei of an individual. In the 

 case of the rod-shaped chromosomes the 

 spindle fibre is attached to one end, and in 

 the case of the bent chromosome it is 

 attached to the angle of the V. Often homo- 

 logous chromosomes differ in this respect, 

 as shown by the shape of the bivalents in 

 ^m^^.TS&^t£'''oftilTot^^^^^ metaphase I. (Fig. 67). A bivalent may 

 (After Carothers, /.M..1917.) ^6 formed by two Straight chromosomes 



(Fig. 67, A) or by two bent chromosomes (B) or by one straight and 

 one bent chromosome (C), all these figures representing the same 

 bivalent as found in three different individuals. 



A number of other cases of inequalities between homologous chromo- 

 somes have been recorded, of which further mention need only be made 

 of Gryllotalpa borealis (Payne, 19 13 a) where the inequality is connected 

 with the sex chromosomes and may be of the same nature as found 

 in the unequal XY pair, which of course constitutes the most striking 

 example of unequal homologous chromosomes.^ 



C. VARIATION IN THE NUMBER OF CHROMOSOMES 



Variations from the number of chromosomes typical for the species may 

 be due to the following causes : 



(a) Transverse fracture of the chromosomes (fragmentation) leading 

 to increase, or end-to-end fusion (linkage) causing decrease, in number. 



^ Boveri (1904) found a female A. megalocephala bivalens in which both the tetrads were 

 often formed of two rods of unequal length. 



