THE DEOXYRIBONUCLEIC ACID CONTENT OF THE NUCLEUS 179 



in the DNA content of epidermal nuclei under the action of croton oil. 

 Some abnormalities reported by Leuchtenberger et al}^^ in the DNA con- 

 tent of certain samples of human sperm are probably related to sterility. 

 Leuchtenberger et al}^^ studying the DNA content of spermatozoa of a 

 large number of men (6,000 cases) showed that while fertile men had always 

 the same characteristic amount of DNA in the spermatozoa, infertile men 

 had significantly lower DNA values. 



Dividing Cells, Mitosis, Meiosis. Pasteels and Lison^'- had considered that 

 nuclei have a higher DNA content in dividing than in nondividing tissues 

 and this increase could not be explained by the occurrence of mitosis. Alfert 

 and Swift^^ demonstrated that in chick fibroblasts in tissue culture the nu- 

 clei in the migrating zone are flattened while in the dividing zone they are 

 spherical. Because of this difference in shape, the calculations of Pasteels 

 and Lison give values for the nuclei of the dividing zone which are 50 % too 

 high. If a proper correction is made for the difference in shape, the values 

 of DNA are in good agreement in the two zones. Alfert and Swift^^ also 

 criticize the data of Pasteels and Lison on cells of the crypts of Lieberkiihn 

 in the rat at different mitotic stages. They found that DNA synthesis oc- 

 curs in interphase some time before prophase, and not in telophase. Errors 

 due to stray light would explain the results of Pasteels and Lison. Roels^^^ 

 also showed that in thyroid cells of the rat stimulated to mitotic division 

 by thiouracil, the synthesis of DNA occurs just before the onset of prophase. 

 Moses and Taylor^^^ reported that the synthesis of DNA occurs in Trades- 

 cantia prior to the evidence of duplication of the chromosomes in early 

 meiotic prophase, late microspore interphase, and early pollen interphase in 

 the generative nucleus, and concluded that DNA synthesis occurs in preT 

 divisional stages. Taylor and McMaster"^ studied by autoradiography the 

 synthesis of DNA in microgametogenesis in Lilium longiflorum. This 

 synthesis occurs before nuclear division. The DNA content of the nucleus 

 during the course of gametogenesis was exactly related with the chromo- 

 some number. Finally, Alfert and Swift^^ found in the annelid Sabelleria 

 amounts of DNA per nucleus in very good agreement with the values ex- 

 pected from the number of chromosomes in the different stages of the 

 maturation of the eggs and of cleavage after fertilization, in contradiction 

 to the earlier results of Pasteels and Lison. Swift and Kleinfeld^^^ had found 



"^ C. Leuchtenberger, F. Schrader, D. R. Weir, and D. P. Gentile, Chromosoma 6, 



61, (1953). 

 "* C. Leuchtenberger, D. R. Weir, F. Schrader, and R. Leuchtenberger, Excerpta 



Med. Sect. 7 8,418 (1954). 

 "" H. Roels, Nature 173, 1039 (1954). 



1" M. J. Moses and J. H. Taylor, Records Genet. Soc. Amer. 22, 88 (1953). 

 "8 J. Herbert Taylor and R. D. McMaster, Chromosoma 6, 489 (1954). 

 1" H. Swift and R. Kleinfeld, Physiol. Zool. 26, 301 (1953). 



