CONTROL OF LENGTH 57 



Matthiola mutation in chromosome length (Ch. X), which affects 

 only the first meiotic division (J. Clausen, 1931 c). 



Table 5 shows the total relative lengths of the chromosomes 

 at mitosis in the haploid sets of three Crepis species and their first 

 generation hybrids (Navashin, 1931 b ; cf. Fig. 7). 



The chromosomes of capillaris are longer, and those of neglecta 

 shorter, in the hybrid than in the parent. It is also found that 

 their bulk is the same, greater length meaning less width. It is to 

 be assumed that the species are genetically different in regard to the 

 control of spiralisation, while the genotype of the hybrid determines 



Fig. 13. — Mitotic metaphases in Crepis capillaris. A. Normal. 

 B. In a root-tip that has been cooled before fixation. This 

 change may be due to a difference in life or to a difference in the 

 conditions of fixation. The difference is parallel to that found in 

 different races of some species. X ca. 2000 (from Delaunay, 1930). 



a uniform width in its chromosomes whatever their width in the 

 parent from which they are derived (cf, Levan, 1936). 



This difference has been imitated in Crepis by cold treatment 

 before fixation (Delaunay, 1929, cf. Fig. 13 ; cf. Appendix II). 



[e) In the neuropteran Hemerohius pirn the chromosomes are 

 longer and slenderer in the female than in the male (Naville and de 

 Beaumont, 1933). 



(iii) Chromosome Aggregation and Diminution. In certain 

 species of Ascaris the fertiUsed eg§ contains a small number of large 

 chromosomes. At the second and later segmentation divisions in 

 those cells which are not going to give rise to germ cells, these 

 large chromosomes break up into numerous small ones which 

 remain independent in all the nuclei derived from them. The 

 change is thus irreversible. In eggs that have been fertilised by 

 two sperms and have divided into three or four cells at the first 

 division instead of two, the number of nuclei with unfragmented 



