CHROMOSOME INDIVIDUALITY 45 



separation is only possible where the resting stage is short and 

 therefore not a complete resting stage. For example, in the 

 pre-meiotic resting stage of the spermatocytes (Wenrich, 1916, on 

 Phrynotettix , Renter, 1930, on Alydiis, et alii) and in the early 

 segmentation divisions of the fertilised e^g (Heberer on Cyclops, 

 1925 ; Smith on Cryptobranchus, 1929) the chromosomes remain in 

 distinct vesicles during the resting stage, from which they reappear 

 at the ensuing prophase. In the spermatocytes of Orthoptera one 

 chromosome (the sex chromosome) is more condensed than its 

 neighbours and stains more deeply, so that it can be distinguished 

 from the others throughout the resting stage. Similarly, in sperma- 

 tozoa, even in vivo, it is sometimes possible to see the individual 

 chromosomes and actually distinguish between those having 

 different numbers (Miilsow, 1912, on Ancyracanthus ; Meves, 1915, 

 on Filaria ; v. male gametes). Kater (1926) thinks it possible to 

 distinguish the separate compartments in which the chromosomes 

 lie during the resting stage in root tip nuclei. 



(iii) Finally, a more elaborate kind of evidence is provided by the 

 comparison of chromosome forms in hybrids with those found in the 

 parents. Hybrids between species with clearly distinguishable 

 complements have been studied in Crepis. It is found that each of 

 the chromosome types of the parental species reappears, so that no 

 two chromosomes are ahke (Fig. 9, HoUingshead, 1930 ; Navashin, 

 1927). The constancy was not maintained in the second generation, 

 for special reasons which do not concern the present problem 

 {v. Ch. VII). This kind of evidence is afforded also by the observa- 

 tions of Avery (1929) on other hybrids of Crepis, Meurman on Ribes 

 (1929), and by some older work. 



(iv) The small resting nuclei of many flowering plants are to 

 varying extents organised differently from the large nuclei on which 

 accurate observation is easiest. They consequently show the 

 permanence of the chromosomes in a different way. Small nuclei 

 have been known to contain deeply-staining bodies, chromocentres 

 or prochromosomes, in the resting stage which were distinguishable 

 from nucleoli and corresponding in number with the chromosomes 

 (cf. Rosenberg, 1909). These bodies have been shown by Doutre- 

 ligne (1933) and others to consist of the proximal parts of the 



