THE INTERPHASE NUCLEUS 



in a human mammary carcinoma (Biesele and Poyner^^'). In the 

 latter, chromosomes of both twice and four times the normal volume 

 were found in metaphase figures all of which were approximately 

 diploid. The nuclei of these tumours are therefore polytene and not 

 polyploid. Three modes of interphase nuclear volume correspond with 

 the three sizes of mitotic chromosomes. 



It must be admitted that the doubts expressed above concerning the 

 accuracy of volumes computed from measurements in microscopical 

 preparations of the order of tens of microns apply with still greater 

 force when one dimension of the object is a fraction of a micron. 

 Biesele is aware of the possibility of error in his methods, and at least 

 it can be said that his camera lucida drawings demonstrate that the 

 chromosomes of tumours can be larger than those of normal cells. 

 Nothing is yet known for certain of the internal structure of such 

 polytene chromosomes. 



It is not only in the malignant cell, however, that chromosomes can 

 vary in size. It has long been known that both intermitotic nuclei and 

 chromosomes decrease in size as the segmentation of the egg is in 

 progress. During the first few cleavages, the total volume of all the 

 nuclei may either increase at each cleavage by some factor less than 2, 

 or remain constant as in the mouse, according to Hertwig (G.).^^^ 

 In Echinoderms, both intermitotic nuclei and chromosomes decrease 

 in size during development, although in the literature of this subject 

 divergent views are expressed on the precise course of these diminu- 

 tions in volume (Erdmann^^^; Godlewski^^"). 



Hertwig (G.) refers to the chromosomes of the fertilized mouse egg 

 as 'polymers' which are progressively halved at each cleavage, without 

 increase in their substance between divisions. On the other hand 

 Alfert^^i finds that the range of relative DNA content of the nuclei 

 of the mouse egg during early cleavage stages is similar to that of the 

 primary oocyte. Again, we have yet no evidence of the internal structure 

 of these chromosomes, which even at their largest are not readily 

 amenable to microscopical investigation. Biesele^^^ has compared 

 the volume of the chromosomes in the liver of the newborn and the 

 full grown rat, and finds that there is a twofold increase in their volume 

 during adult life. It would be of great interest to extend such studies 

 backwards into foetal life, and to compare the size of the chromosomes 

 and of the interphase nuclei with their content of nucleic acids in 

 absolute terms. 



isolation of chromatin threads from resting nuclei 



The development of fractionation methods for the isolation of cell 

 constituents in an almost undamaged condition has been one of the 



63 



