THE DEOXYRIBONUCLEIC ACID CONTENT OF THE NUCLEUS 173 



chambers of the ovarial lobes of one of the Hemiptera, described extrusions 

 of DNA out of the nucleus in some cells as participation of this DNA in 

 the formation of nutritive substances which are transferred to the eggs. 

 The very detailed description of these authors shows that the considerable 

 increase of the DNA per nucleus (up to seventeen times) results probably 

 from the fusion of several nuclei (instead of the process of endomitosis 

 suggested by Lison and Fautrez-Firlefyn), Finally, Leuchtenberger and 

 Schrader** have found in the salivary gland of Helix aspersa a striking ex- 

 ample of considerable variation of the DNA content per nucleus (from 30 

 to 1) correlated with the production of secretory products in the cytoplasm. 

 The considerable variations of the DNA content of the nucleus reported 

 here seem to be rather rare. They have been described only in insects and 

 lower organisms; nothing similar has been found in vertebrates. When these 

 processes do not represent some degree of polyploidy, they can be inter- 

 preted as a particular case of direct participation of the DNA of the nucleus 

 in the secretory process of the cell. The enormous synthesis of DNA in such 

 nuclei, which are not going to divide, is of great interest and could throw 

 some light on the physiological role of the nucleus. 



3. The DNA Content of the Nucleus in Dividing Cells 

 a. Preparation for Mitosis and Meiosis 



We must now consider the problem of the DNA content of dividing 

 nuclei. Since the two daughter cells have the same amount of DNA in 

 their nuclei as the mother cell, cell division must be preceded or accompa- 

 nied by a synthesis of DNA. When does this synthesis occur? The exact 

 moment is still a matter of argument. The earliest observations come from 

 Caspersson's ultraviolet work*^ on the spermatogenesis of the grasshopper. 

 They suggested that the synthesis of DNA occurs during the first phases of 

 division and reaches its maximum at metaphase. Ris,^° on the other hand, 

 studied mitosis in terminal meristems of onion root tips and spermatogene- 

 sis of Chorthophaga by the photometric technique in visible light and ob- 

 tained results in agreement with Caspersson's: the synthesis of DNA would 

 occur at prophase and would be complete at metaphase when the total 

 amount of DNA of the chromosomes was twice that of a normal nucleus. 

 But more recent work indicates that the synthesis of DNA must occur earl- 

 ier. When the nucleus begins its morphological changes characterizing pro- 

 phase, it should already show the DNA content of the two future nuclei. 



It should be pointed out that a number of authors^^' ^^- '"' have shown 

 that, in meiosis, the first spermatocyte already contains twice the amount 

 of DNA of the diploid nuclei, i.e., all the DNA necessary for the four sperma- 



«8 C. Leuchtenberger and F. Schrader, Proc. Natl. Acad. Sci. U. S. 38, 99 (1952). 



*' T. Caspersson, Chromosoma 1, 147 (1939). 



so H. Ris, Cold Spring Harbor Symposia Quant. Biol. 12, 158 (1947). 



