THE MITOTIC CYCLE 



most Striking advances in experimental cytology. These procedures 

 have been applied both to the cytoplasm and to whole nuclei, methods 

 for the isolation of which have long been known (p i). The final 

 achievement of these techniques seemed to be the subdivision of the 

 nucleus itself from which it has been claimed that veritable chromo- 

 somes can be prepared in bulk. This further step, however, involves 

 more than the separation of bodies already present in the cell, for the 

 initial material is the interphase nucleus. A claim that the end-product 

 consists of isolated chromosomes must therefore be examined with some 

 care. 



The first accounts of nuclear fractionation were given by Claude^^^ 

 and by Claude and Potter. ^^^ The material used by these authors 

 is the spleen of leucaemic mice, which is completely infiltrated by 

 lymphocytes, mainly haemocytoblasts. The tissue is first frozen and 

 then ground with sand at 5'' C. and the chromatin threads afterwards 

 are separated by differential centrifugation. The final product consists 

 of Feulgen-positive threads, either single or paired O'5-i -Ofx in diameter. 

 As to the nature of these threads, the authors' view may be quoted : 

 'Their typical arrangement in pairs with similarities in width and bead- 

 ing suggest that these threads may correspond to the chromosome 

 strands as they occur in the nucleus during the resting stage,' It is 

 clear from the authors' illustration that the interphase leucaemic nuclei 

 are very markedly of the 'thread type', as Serra^^^ terms such nuclei 

 with much residual chromatin. However, Claude and Potter state 

 further that, 'similar material has been obtained from normal tissues, 

 for example guinea pig or rat liver'. 



The claims of Mirsky et alii are more outspoken. Their material is 

 isolated by means of the Waring blender from fish and bird erythrocytes 

 and from mammalian tissues (Pollister and Mirsky^^c) ; and again we 

 may quote the description of the microscopical appearance of threads 

 prepared by this method (Mirsky and Ris^^^^ p ^) : 



They vary greatly in size and organization. Usually their doubleness can be 

 clearly seen. They show a specific pattern with trabants, heterochromatic and 

 euchromatic sections. Like chromosomes they can be uncoiled by certain agents, 

 KCN for example. These characteristics by themselves are strong evidence for 

 the chromosome nature of these threads. Final proof, however, lies in the repeated 

 occurrence of the same type of chromosome. 



Such a remarkable claim we must attempt to discuss. In the first 

 place, as Mirsky and Ris227 point out, their threads differ in form from 

 those of Claude and Potter, for they are shorter and more coiled 

 than those of the latter authors. The nitrogen and phosphorus content 

 of both types of threads are, however, very similar (Mirsky and Ris^^?). 



If these threads are really chromosomes, they have been formed 

 from an interphase nucleus, which, subjected to violent mechanical 



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