vi] THE MEIOTIC PAIRING 85 



the set of chromosomes introduced into the zygote from 

 the male parent A, B, C, D, . . . , and the corresponding 

 maternal set A', B' ', C', D', . . . , the somatic or diploid series 

 consists of A, A' , B, B', C, C', . . . , usually mingled indis- 

 criminately together 1 . When, however, the meiotic phase in 

 the maturation of the germ-cells supervenes, the chromo- 

 somes pair together, A with A' ', B with B', and so on, so 

 that each bivalent in the heterotype division consists of 

 two homologous chromosomes, one of which is of paternal 

 and the other of maternal origin. Since the two of each 

 pair are commonly alike it is not possible to say from direct 

 observation whether all the paternal chromosomes go to 

 one pole of the spindle, and all the maternal to the other, 

 or whether they behave irregularly, but a comparison of the 

 chromosome behaviour with the facts of heredity leads to 

 the definite conclusion that in respect of any pair it is a 

 matter of chance whether one or the other member goes to 

 a given pole. This conclusion is confirmed by observations 

 such as those of Miss CAROTHERS (1917) on the rather rare 

 cases in which the members of a pair may differ in size or 

 in the position at which the spindle-fibre is attached. 

 Miss CAROTHERS found that in a grasshopper the spindle- 

 fibre may be attached at the end of the chromosome, giving 

 a rod-shape in anaphase, or nearer the middle giving a J- 

 shape. In the spermatocytes of some individuals rods and 

 J-forms were paired together, She then found that there 

 was no regularity in the arrangement of these pairs on the 

 spindle, but that the segregation of the members of different 

 pairs was quite independent of one another. It must there- 



1 In the Diptera in all the cells of the body, whether somatic or germinal, 

 which have the diploid number, homologous chromosomes constantly lie next 

 to one another in the equatorial plate, A being associated with A', B with B', 

 etc. (METz). This is not usual in other groups of animals. 



