THE MITOTIC CYCLE 



first detectable at the morula stage. They are everywhere well developed 

 in the gastrula. The cells of the presumptive endoblast do not become 

 basophilic until much later, while on the other hand, those of the 

 neural ectoderm early show a marked basophilia, which is later lost, 

 but the nucleoli then remain equally prominent. It was suggested by 

 PoLLiSTER and Leuchtenberger^'* that the high proportion of oligo- 

 nucleotides which they found in the nucleoli of pollen mother-cells of 

 ^ea mais indicated that synthetic activity was in progress therein, 

 though it could equally well be argued that this fact suggests depolymer- 

 ization heralding the approaching break-up of the nucleolus. It would 

 be valuable to compare the proportion of oligonucleotides in nucleo- 

 proteins at other sites where they are believed to be actively concerned 

 in synthesis, such as the cytoplasmic nucleoproteins, and the hetero- 

 chromatic regions of the salivary chromosomes of the Drosophila larva. 



Isotopic methods 



By the use of isotopic methods, evidence has been obtained which 

 suggests that the ribonucleoproteins of the cell can originate within the 

 nucleus. Living cells and tissues have been treated in various ways with 

 phosphates in solution, containing P^^; these cells have been subse- 

 quently fractionated and the specific radioactivities of the various 

 fractions compared. Marshak and Calvet^^ and Jeener and 

 SzAFARZ^^ have used mammalian liver for such experiments, while 

 the two latter authors have also worked with mice embryos, and cul- 

 tures of the flagellate Polytomella. All agree that at first the activity of 

 the nuclei is greater than that of the cytoplasm. Jeener and Szafarz 

 separated their cytoplasmic material into fractions of graded sizes of 

 particles, and found that the smallest took up radiophosphorus more 

 quickly than did larger particles. Their conclusion is that in the 

 nucleus is synthesized at least an important part of the RNA of the 

 cell, and that this passes through the nuclear membrane in an extremely 

 fine form (unsedimentable at 60,000 x g), and then 'spreads into the 

 rest of the cytoplasm by integrating itself with cytoplasmic particles of 

 large dimensions'. 



While more remains yet to be learnt regarding the interrelationships 

 in both space and time within the cell between basic and non-basic 

 proteins and the two types of nucleic acid, the general thesis that some 

 connexion exists between the function and formation of these substances 

 in all cells may meanwhile be accepted, although nothing is yet known 

 of the steps by which proteins are synthesized in the living cell. It has 

 been suggested that nucleic acids may furnish phosphate groups to an 

 intermediate substance in which energy-rich phosphate bonds are 

 built up, which in its turn could provide the necessary calories for the 

 synthesis of peptide bonds by phosphorylation. The association of 



16 



