312 CHROMOSOMES IN HEREDITY: PHYSIOLOGICAL 



rat at metaphase inasmuch as these chromosomes frequently He off 

 the plate or on the edge of it. 



The possibility of an embedding of sex chromosomes in the 

 nucleolus which is suggested by these observations in Orthoptera 

 and Mammalia is in keeping with their observed association with 

 nucleoli, of the kind found in plants, in meiosis of many animals 

 which do not show the ordinary differential condensation of sex 

 chromosomes. Thus the X chromosomes instead of being themselves 

 condensed are associated with the nucleolus in oogenesis and 

 spermatogenesis in Lepidoptera, in oogenesis in Orthoptera (Buchner, 

 1909) and in mitoses in Drosophila (Table 45). Against this view 

 two objections must be taken into account. First, sex chromosomes 

 that are equally precocious in mitosis and meiosis are attached to 

 the nucleolus in some Hepaticae, but not in others (Tatuno, 1933). 

 Secondly, the X chromosome in Stenobothrus appears equally con- 

 densed at diplotene with the specific Feulgen treatment (Bauer, 

 1932). This stain is not, however, necessarily capable of distin- 

 guishing extraneous materials that are in close association with 

 chromosomes. When we recall that the nucleolus can be con- 

 densed on the chromosomes under special physiological conditions 

 in plants where it is normally organised at special centres, we see 

 that precocious condensation is plausibly explained as the result of 

 abnormal nucleolar formation in certain cases. 



The general character of this behaviour makes it clear that we 

 are dealing with a reaction of particular chromosomes to special 

 genetic conditions rather than with the reaction of dissimilar 

 homologues with one another in pairing, such as is characteristic of 

 structural hybrids. Further critical evidence is afforded by the 

 following observations. 



{a) The two X chromosomes in the hermaphrodite generation 

 of Rhahditis nigrovenosa condense precociously in oogenesis. In 

 spermatogenesis in the same individual this precocity is perhaps 

 exaggerated, since they fail to pair. 



{b) The compound unpaired X chromosome of male Perla 

 marginata (Plecoptera) behaves differentially in spermatogenesis, 

 but not differentially in meiosis in an ovary-like organ produced by 

 the male (Junker, 1923). 



