54 Nature of the Genetic Material 



nicrling), and transferred into the cytoplasm of cells in which protein 

 synthesis is taking place. RNA, therefore, is considered to be im- 

 portant mainly for protein synthesis. It occurs in the chromosomes 

 also, which might mean that it is itself synthesized there. Its quantity 

 is not constant. (See, above, the "elimination chromatin.") However, 

 there are many indications that DNA is derived only from former 

 DNA. There is a huge body of facts in the older literature showing 

 that growing oocytes will assimilate other nuclei. That this means 

 mainly DNA is demonstrated by such cases as that discovered in 

 some Bryozoa by Bonnevie (1907). Here the growing oocyte is in- 

 seminated by many spermatozoa which are dissolved in the cytoplasm 

 obviously serving as source of DNA. In archiannelids (Dinophilus; 

 see Shen, 1936) and some oligochaetes (Oschmann, 1914, and 

 others), nurse cells are phagocyted by the growing oocyte, and a 

 series of nuclei (i.e., their DNA) enter the cytoplasm of the oocyte. 

 In obligatory polyspermy ( Lepidoptera, Selachia), there is at least 

 a suspicion that the introduced extra nucleic acid is used up in some 

 way, which might mean as source material for DNA in the nuclei. 

 Though biochemical work has not yet been done on all these cases, 

 the facts are highly suggestive. 



Nurse cells which are not incorporated into the ovum but become 

 loaded with DNA by endomitosis have been described for the insect 

 ovary by Painter and Reindorp (1939) and for tapetum cells of 

 anthers by Brown (1949). In these examples, as well as some in 

 which no polyteny is known, it has been observed that DNA is 

 liberated from the nurse cell nuclei and enters the cytoplasm of the 

 egg cell with the nutrient stream (Schrader, 1951, for insects; 

 Konopacki, 1936, for the ascidian egg). Still more important is the 

 observation by Cooper (1952) that DNA granules leave the tapetum 

 cells of the anthers and enter bodily the nuclei of the microsporocytes 

 during the synaptic stages with polar orientation of the chromosomes, 

 and spread over the chromosomes from their ends. In the light of 

 these results some of the old material, in which nucleo-cytoplasmic 

 exchanges during the bouquet stage have been claimed or doubted 

 (Blatta spermatocyte, axolotl oocyte), should be checked again after 

 more than forty years. 



In this connection, also, the changing views on cytoplasmic DNA 

 should be recorded. It used to be a dogma that DNA does not occur 

 in the cytoplasm. Hoff-j0rgensen and Zeuthen (1952) and Elson and 

 Chargaff (1952), among others, found enough DNA in the cytoplasm 

 of sea urchin eggs to take care of the needs of the embryonic nuclei. 



