THE RESTING STAGE 43 



is optically homogeneous or granular, and considers this to be 

 inconsistent with the view that any threadlike structure is present. 

 The granular structure that is sometimes found in ftxed nuclei 

 might be given by an optical section of a coil of threads, but it 

 certainly does not always correspond to such a coil if the fixation is 

 inefficient, because, as Belar points out (1928, Plate XIV, cf. Appendix 

 I), the granulation is coarser the slower and more inefficient the 

 fixation. Micro-dissection work has similarly suggested that the 

 nucleus has no permanent structure and that its contents are 

 " liquid " in vivo (Chambers, 1925). But failure to detect structure 

 by these methods is not conclusive {cf. Heilbrunn, 1928). The 

 structure that is sought is without analogy on a larger scale or in 

 other materials. 



What we have already seen, on the other hand, of artificial 

 resting nuclei and of the continuance of the spiralisation cycle of 

 the chromosomes from telophase in the next prophase can leave 

 no doubt either of the structural relationship of the resting nucleus 

 and the metaphase chromosomes or of the structural stability of 

 the chromosomes in the nucleus. It is worth while, however, to 

 consider some of the particular evidence of the permanence of the 

 chromosomes during the resting stage since this principle is the 

 foundation of chromosome genetics. 



(i) The classical example was described by Boveri (1909), again 

 in Ascaris. He showed that each of the several types of relative 

 arrangement of the chromosomes seen at the beginning of a division 

 in a young embryo corresponded exactly with one of the several 

 types seen at the end of the previous division. Parts of the chromo- 

 somes, therefore, reappeared in just those parts of the nuclei where 

 they had disappeared. 



The same kind of evidence, but yet more striking, is afforded by 

 the observations of Belar on Aggregata (1925, 1926). Here, owing 

 to the rapid succession of divisions in the sporozoite, the longer 

 chromosomes remain entangled at the ends at telophase {v. supra). 

 They reappear entangled at the next prophase, and after two or 

 three divisions the entanglement seen is precisely such as would 

 have been expected had there been no resting stages. Similarly, 

 Belar (1929 b) showed in living mitosis in the staminal hairs of 



