CONTROL OF SIZE 



55 



The following examples show the range of genotypic control seen 

 at mitosis. 



(i) Chromosome Volume, {a) Tradescantia hrevicaidis is a triploid, 

 apparently a hybrid between tetraploid Tradescantia virginiana 

 with large chromosomes and a diploid relative with smaller 

 chromosomes of proportionate diameter. It has chromosomes 

 intermediate in length and width between those of its supposed 

 parents. In the pollen grains af one bud of the plant studied the 

 chromosomes were proportionately reduced to one-fifth and the 

 resting nuclei to one-third of the normal size (Fig. 12). This was 

 not associated with any change in the size of the cell (D., 1929 c, and 

 unpublished ; cf. Thomas, 1936, on Lolium, Fig. 122). 



{b) The chromosomes of Vicia sattva and Vicia angustifolia are 

 smaller in the hybrid than in the parental species, at mitosis and 

 meiosis (Sveshnikova, 1929, a and h). Similarly Fg hybrids from 

 Dianthus monspessidamis x D. plumariiis have chromosomes 

 uniformly smaller than either of the parental species at mitosis 

 (Rohweder, 1934). 



(c) In Sphcerocarpus Donnellii the chromosomes as a whole and the 

 cells are 17 times larger in the female than in the male. This is 

 largely accounted for by the presence in the female of the large 

 X chromosome instead of the small Y chromosome, but partly also 

 by the greater size of the other chromosomes. Apparently a genetic 

 factor controlling chromosome size is correlated with sex in this 

 species (Lorbeer, 193c). In man and the rat the chromosomes are 

 longer (and probably proportionately broader) in the male than in 

 the female (Evans and Swezy, 1928, cf. Table 12). Similarly the 

 chromosomes (or at least the X chromosome) are larger in the male 

 Melandrium than in the female (Belar, 1927). Witschi (1935) 

 finds that the chromosomes are four times as large in the oocytes 

 as in the spermatocytes of hermaphrodite Lepas anatifera (Cirri- 

 pedia). In this case differentiation has the same effect as has a 

 genetic difference in other cases. 



(d) A particularly significant change is found in haploid Triton 

 embryos, where the chromosomes are larger than in diploids (Fank- 

 hauser, 1934 h). 



These changes in bulk are paralleled by the effects of differences 



