CELLS IN DIVISION 



described by White^^s ^^^d Mickey/^* and in Ambystoma by 

 Creighton.^^^ Among Protozoa with a small number of large chromo- 

 somes, their spiral structure is exceptionally clear, as for instance 

 in Collozoum (Patau^^^) ) and such flagellates as Holomastigotoides 

 (Cleveland^"* ^^9) (Figure 33). Cleveland's recent paper is one of the 

 most remarkable papers on nuclear cytology ever to be published. This 

 organism and the related Protozoa offer unique advantages for the 

 study of nuclear cytology which this author is exploiting to the full. 



Minors qf chromatids 

 26e 



Figure 33 Anaphase in Holoma- 

 stigotoides tusitala x 2100. From 

 Cleveland^"* {By courtesy, Trans. 

 Amer. Phil. Sac). Nucleus showing 

 major and minor coils in chroma- 

 tids. Compare with Figure 31. 



Their number of chromosomes is either two or three and they can be 

 recognized individually throughout their life cycle. 



Chromosomes in prophase, at stages before contraction has begun, 

 often show a banded structure, in which there are irregularities in 

 density and cahbre along the course of the chromonemata; these denser 

 segments are known as chromomeres. It is debated whether these 

 chromomeres are in reality coiled regions of a uniform chromonema 

 which elsewhere is nearly uncoiled; this point of view is maintained by 

 j^jsi3o ^}^Q denies that chromonemata are ever seen in a fully uncoiled 

 condition. Ris quotes a number of instances where structures described 

 as chromomeres have later been found to be coils in chromonemata, 

 and maintains that this is true of the leptotene chromosomes of the 

 Orthopteran spermatocytes, which have previously been regarded as 



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