164 R. VENDRELY 



tissue) was claimed to be markedly lower than the expected value (twice 

 that of the spermatid). A similar examination of rat tissues was made by 

 Leuchtenberger et al}^ and also by Ris,^^ q;^^ by Swift'* with Pollister's 

 apparatus, all of whom found none of the discrepancies reported by Pas- 

 teels and Lison. The apparent discrepancy has now been explained in a 

 recent publication by Pasteels and Lison,'*^ who, in a new series of measure- 

 ments made on other rats (their first experiment was on only one animal), 

 have obtained results in agreement with the cytochemical and chemical 

 observations of other workers. The single rat studied previously by Pas- 

 teels and Lison appears to have been exceptional. A complete study of such 

 a case would be of the greatest interest, but unfortunately no other rat of 

 this type has been found at the present time. 



c. Quantitative Photometry in Ultraviolet Light 



The absorption of ultraviolet light as in Caspersson's apparatus (Chap- 

 ter 17) has been used recently for a quantitative estimation of the DNA of 

 nuclei."*' The absorption of light at 260 m/x in the nucleus is due to the DNA 

 of the chromatin and also to the PNA of the nucleolus and to their nucleo- 

 tides, but the error caused by these last two groups in the measurement of 

 DNA does not seem to be very high. If the nuclei are fixed, most of the 

 fixatives dissolve simple acid-soluble nucleotides; and if the nuclei are iso- 

 lated, the isolation procedure with citric acid removes a great part of the 

 PNA. Nevertheless, Frazer and Davidson'*'' showed that crystalline ribo- 

 nuclease removes a considerable proportion of the total material absorbing 

 ultraviolet light, so that absorption measurements are much more precise 

 after the use of the enzyme. In any case, photometry in visible or in ul- 

 traviolet light cannot reach a high degree of precision, and a certain 

 amount of error is unavoidable. 



Up to the present time, few results on nuclei have been obtained with 

 photometry in ultraviolet light. Leuchtenberger et aU^ studied isolated nuclei 

 from beef liver, calf thymus, and bull sperm; the results calculated in abso- 

 lute amounts of DNA are in reasonable agreement with the values from 

 chemical estimation. In rat liver, the three classes of nuclei were found as 

 with photometry in visible light. Walker and Yates^"- ^^ reported results 

 obtained on living cells in tissue culture and on living erythrocytes and 

 sperm. Their figures are in agreement with chemical results. Frazer and 

 Davidson,"** working on nuclei isolated from rat kidney and rat liver, chose 



^2 J. Pasteels and L. Lison, Covipt. rend. 236, 236 (1953). 



** C. Leuchtenberger, R. Leuchtenberger, C. Vendrely, and R. Vendrely, Exptl. Cell 



Research 3, 240 (1952). 

 4^ S. C. Frazer and J. N. Davidson, Exptl. Cell Research 4, 316 (1953). 

 *» P. M. B. Walker and H. B. Yates, Proc. Roy. Soc. (London) B140, 274 (1952). 

 " P. M. B. Walker and H. B. Yates, Symposia Soc. Exptl. Biol. 6, 265 (1952). 



