48 S. S. COHEN 



not valid for embryonic tissues. As noted above, it is difficult to assess the 

 relationship of this variabihty to the stage of DNA doubluig and differen- 

 tiation in which the cell has been examined. This case, among others, has 

 been discussed in some detail by Vendrely (1955). 



The analyses of these instances of apparent variabihty has inevitably been 

 tied to the problem of the time of DNA doubling during the mitotic cycle. 

 Most workers (Thorell, 1955) have pomted to the interphase for this event, 

 which, as noted earher, occurs simultaneously with histone doubling (Bloch 

 and Godman, 1955). However, Vendrely (1955) has recorded a number of 

 exceptions to this time of DNA doubhng, involving any time from late 

 prophase in onion tip mitosis to late telophase in several vertebrate cells. It 

 may be noted that a far more complicated series of patterns of DNA synthesis 

 and divisions occurs in the development of the pollen grain (Ogur et al., 

 1951). 



It might be imagined that changes in nuclear composition might occur 

 during intense physiological activity, as in the development of the secretory 

 activity of various glands. In the growth of the salivary glands of adult 

 Drosophila melanogaster, the ceUs double in size but do not divide. In this 

 system, the DNA content and volume of the nuclei do not change, although 

 the RNA and protein contents of the cells increase markedly (Patterson and 

 Dackerman, 1952). Such a pattern appears to be common; however, the 

 following remarkable case has been reported: in the salivary glands of the 

 snail. Helix pomatia, the secretion of j)olysaccharide into the cytoplasm is 

 accompanied by a 30-fold decrease of nuclear DNA and nuclear volume 

 (Leuchtenberger and Schrader, 1952). 



That the DNA content of adult nuclei is usually a constant character of 

 the species is supported by considerable evidence. It was hoped that this 

 value would prove a useful taxonomic tool in studying evolutionary relation- 

 ships. This expectation has not been realized, following the accumulation of 

 data on this parameter in a wide variety of organisms (Allfrey et al., 1955b; 

 Vendrely, 1955). Among the invertebrates, there does seem to be a general 

 drift to more DNA per cell in the higher phylogenetic categories, although 

 numerous exceptions to this trend can be found. Among the vertebrates, not 

 even a drift of this sort can be detected. If anythmg, the series (Dipnoi, 

 Amphibia, Reptilia, and Aves) is accompanied by a drop m DNA per 

 nucleus. The range of values within certain classes, such as birds and 

 mammals, is not very great. However, among the Amj)hibia the range may 

 be 20-fold, as in the case of the toad, with 7 X 10~^ mg. DNA per nucleus, 

 to Amphiuma with 168 X 10~^ mg. DNA per nucleus. Mirsky and Eis (1949) 

 have concluded that from species to species the DNA content per cell 

 follows the size of the ceU more closely than the number of presumed 

 genetic units. 



