THE INTERPHASE NUCLEUS 



MiRSKY and Ris^" state that 'observations on egg nuclei show that the 

 quantity of deoxyribonucleic acid in the egg nucleus corresponds very 

 closely to that in the sperm', though no details relating to this alleged 

 equality are given. For the sea-urchin egg this conclusion is not sup- 

 ported by the available data (Schmidt et alii,^* Vendrely and Ven- 

 DRELY^^) which suggests that the fertilized egg ofArbacia contains several 

 hundred times the amount of DNA contained in the single sperm. The 

 Vendrelys do not attach much significance to their result for the DNA 

 content of the egg; but the importance of this question would warrant 

 an extended comparison of eggs and sperms in different animals in this 

 respect. Mirsky and Ris^°* are of opinion that analysis of the sea-urchin 

 egg for DNA by the customary method of Schmidt and Thannhauser 

 must give fallacious results, for a very intense Feulgen reaction would be 

 given if the DNA content were as high as chemical analysis suggests. 

 They state, moreover, that the egg nucleus of Echinometra is Feulgen- 

 negative, and that in Ascaris megalocephala the reactions given by egg and 

 sperm are equal in intensity. It is not clear at present whether this 

 apparent incertitude of chemical assay for DNA is confined to the sea- 

 urchin egg. No adult tissue of the sea-urchin has yet been analysed for 

 tiie nuclear content of DNA, so diploid and haploid nuclei cannot yet be 

 compared in the way which Mirsky and Ris have done for several verte- 

 brates. The immature gonad would probably be the most suitable 

 material for this assay. In relative measurements on Feulgen-stained 

 eggs and larvae oi Paracentrotus lividus, Lison and Pasteels^^^ find that 

 the DNA content of the nuclei vary with their stage of development and 

 from tissue to tissue. Up to the blastula stage, the embryonic nuclei 

 contain 4-5 times as much DNA as does the sperm nucleus. InSabellaria, 

 Pasteels and Lison^® find that the egg and sperm do not contain the 

 same amount of DNA, and that in the fertilized egg, the DNA content 

 of both pronuclei increases markedly prior to their fusion. 



Measurement of DMA content 



One method of measuring the DNA content of a nucleus is to make 

 micro-colorimetric estimations in sections of tissues, either in the ultra- 

 violet, or at visual wavelengths after staining by suitable cytochemical 

 methods. The measured coefficient of extinction can be related to 

 DNA content by an empirical calibration based on the results of bulk 

 estimations. Pollister and Ris^^ claimed that a very similar result for 

 the nucleic acid content of the nucleus of the calf thymocyte was 

 obtained both by ultraviolet measurements and by bulk chemical 

 analysis. Their value for this, however, was less than one-sixth of that of 

 later measurements, and Ris has subsequently admitted that an error 

 was made in calculations from the data for bulk analysis (Leuchten- 

 berger^'). 



LIBRARY 



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