V NUMBER OF CHROMOSOMES 139 



(6) Irregularities of mitosis, by which the chromosomes are unequally 

 distributed between the daughter nuclei, or certain of them are left out 

 of either nucleus. 



(c) Ordinary longitudinal fission of the chromosomes without a 

 mitosis to separate the daughter chromosomes, thus leading to a doubhng, 

 quadrupling, etc., of the number. Or longitudinal fusion of the chromo- 

 somes as in parasyndesis leading to an apparent halving of the number. 



(i) Variation in Chromosome Number due to Fragmentation or Linkage 



Since, as we have seen, the chromatin units are arranged in hnear 

 series in the chromosomes, the total chromatin content of the nucleus 

 may be considered as ideally arranged in such a linear series along a 

 single long thread, which becomes divided into a number of segments 

 varying in number in different species. 



Thus in the genus Cyclops (Braun, 1909) it may be segmented into 

 3(C gracilis), 5 (C. vernalis), 6 (C. viridis), 7 {C.fuscus), 9 (C. hicuspidatus) , 

 or II (C. strenuus), these being the haploid numbers. 



The older cytologists were indeed of opinion that this segmentation 

 of a single linear series actually occurred in the prophase of every mitosis, 

 the first stage in this process being the formation of the " continuous 

 spireme," which in later prophase gave place by transverse segmentation 

 to the " segmented spireme." Though a continuous spireme probably 

 does not occur, at any rate as a regular stage, in prophase (see p. 9), 

 the process probably represents substantially the method by which in 

 evolution the varying chromosome numbers have been produced. 



A special study of the phylogenetic derivation of chromosome numbers 

 has recently been made by American cytologists. The number of chromo- 

 somes in the grasshoppers [Orthoptera) is relatively constant. Thus in 

 ten species belonging to five genera of the family Tettigidae, Robertson 

 (1916) found in every case 2^ = 13 (male) and 14 (female). In over 

 forty genera of Acridiidae, zn is 23 (male) and 24 (female) in all except 

 three, of which one {Chorthippus) has 17 and the other two from 20 to 

 24 chromosomes. 



Robertson {loc. cit.) has shown how the smaller number in Chorthippus 

 {= Stenobothrus) has probably been derived from the type number for 

 the family. The Acridiid spermatogonial chromosome is typically rod- 

 shaped. In Chorthippus, however, only eleven of the chromosomes, 

 including the X chromosome, are of this shape, the remaining three 

 pairs being V-shaped. The angles of the V's — that is to say, the points 

 of junction of the Umbs — are marked by a constriction or non-staining 

 bridge between the hmbs. Robertson makes the very reasonable sugges- 

 tion that the V's have been formed by association or linkage of couples 



