CELLS IN DIVISION 



Further investigation of small metaphase chromosomes by means of 

 the electron microscope is greatly desirable, especially in combination 

 with alkaline treatment. Most of the papers on this subject are as yet 

 on a preliminary scale (Elvers ;i^^ Buchholz^^^). The electron micro- 

 graph of a human pachytene chromosome published by Schultz 

 et alii^^^ suggests a chromomeric structure. In a recent study of this 

 kind by Rozsa and Wyckoff/^^ onion root meristems were sectioned 

 sufficiently thinly to show nuclear structure. Neutral fixation in formalin 

 gave the best results, but showed detail in neither the spindle nor the 

 chromosomes, which to quote these authors 'do not present a definite 

 fine structure at any order of magnitude visible in the electron micro- 

 scope'. HovANiTZ^^* isolated 'chromosomes' from chick erythrocytes 

 by the method of Pollister and Mirsky,^^^ and found a suggestion of 

 spiral arrangement in electron micrographs of this material; similar 

 threads prepared from mammalian nuclei by Mirsky^^^ are clearly 

 coiled, but since those prepared by a different method by Claude and 

 Potter^ ^' consist of straight fibres, it is probable that the revolving 

 blades of the Waring blender are the source of these particular spirals. 



COMPOSITION OF CHROMOSOMES 



We come now to the important subject of the changes in chemical 

 composition of the chromosomes during the mitotic cycle. So far, those 

 researches which bear on this problem have all used cytochemical 

 methods ; it would be desirable to employ bulk methods by the analysis 

 of batches of simultaneously fertilized eggs, but the sea-urchin egg, the 

 most accessible material for such an investigation, has a very low 

 content of DNA in proportion to its ribonucleoproteins (Schmidt et 

 alii,^^^) and nuclear changes over the period of the cleavage cycle would 

 probably not be revealed with sufficient accuracy. 



Nucleic acids 



We shall first consider the question of the nucleic acids during cell 

 division. Inspection of Feulgen preparations of tissues in which mitoses 

 are frequent suggests that a nucleus in division contains very much 

 more Feulgen-positive material than one in interphase; quantitative 

 measurements, however, do not confirm this. The apparent difference 

 between a daughter group of chromosomes in anaphase and an inter- 

 phase nucleus is in part due to the effect of the dispersal of material in 

 the latter through a wider area ; the importance of this factor cannot be 

 assessed by subjective impressions alone.* 



The first measurements which relate to the changes in composition 



* Leuchtenberger*** has shown that by measurements on pycnotic nuclei, their intense 

 reaction towards the Feulgen reagent does not apparently indicate any increase in the 

 amount of DNA therein. 



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