24 MITOSIS: THE CONSTANCY OF THE CHROMOSOMES 



half-chromosomes) correspond in every way at the stages when they 

 occur in the two types of nuclear division there can be no objection 

 to calling a half-chromosome a chromatid both at meiosis and 

 at mitosis. 



Following their appearance the chromosomes contract lengthwise, 

 and to a slight extent in gross bulk. They are evidently more 

 rigid owing to loss of water, and they appear to suffer less 

 from the action of fixatives. During the resting stage the nucleus 

 has probably been uniformly filled with the material of the chromo- 

 somes and with water associated with them. Loss of fluid by the 

 chromosomes during prophase therefore merely means that the 

 nucleus becomes differentiated into chromosomes and " nuclear 

 sap." 



It was formerly held that at this stage the chromosomes were 

 united end to end to form one " continuous spireme." It has now 

 been shown in all clear cases (and especially at meiosis, on which 

 great attention has been concentrated) that this is not so (cf. 

 Fig. 4). Since ring chromosomes have been discovered (Ch. Ill) 

 this structure is inconceivable. Evidently the free ends of chromo- 

 somes are particularly liable to collapse with defective treatment 

 {v. Appendix I). The assumption of a continuous spireme is there- 

 fore unjustifiable on cytological grounds. On genetical grounds it 

 is equally unjustifiable, for the constant production of a single 

 continuous ring requires the secondary assumption of a temporary 

 specific attraction between dissimilar pairs of ends of chromosomes. 

 This assumption is contrary to the principles of chromosome 

 mechanics (v. Ch. XII) and breaks down when applied to hybrids 

 and polyploids and cases of interlocking at meiosis (q.v.). A 

 genuine continuous spireme probably occurs under special conditions 

 in the coccid leery a purchasi (Hughes-Schrader, 1927). The 

 chromosomes now usually have a uniform thread structure. But 

 in certain nuclei (especially at the prophase of meiosis) a granular 

 structure can be seen. Each chromosome then consists of a string 

 of characteristic particles of unequal sizes at unequal distances 

 apart. These particles or chromomeres as we shaU see later have a 

 permanent linear order. 



This is most clearly observed at meiosis (cf. Wenrich, 1916 ; Gelei, 



