210 NUCLEAR TRANSPLANTATION 



action of the hybrids was at first intermediate between that char- 

 acteristic of the parent strains; however, after a few divisions the 

 nucleocytoplasmic hybrids assumed and maintained for over 400 

 cell generations the characteristics of the host cytoplasm, thereby 

 showing that morphological form and locomotion are almost en- 

 tirely independent of the nucleus. When hybrid antigenic charac- 

 ters were tested they were found to be predominantly of the type 

 characteristic for the species contributing the nucleus. Danielli 

 (1955) suggested that these data may be interpreted as evidence 

 that the nucleus is the controlling agent in synthesis of specific 

 macromolecules, but that the cytoplasm is relatively autonomous 

 with regard to supramolecular organization of these molecules 

 within the cell. If this interpretation be valid, one is faced with 

 a most interesting problem concerning the material basis for this 

 cytoplasmic control of growth products which are apparently 

 largely nucleus-dependent in origin. 



As illuminating as these studies are, protozoan material is 

 nevertheless inherently incapable of throwing much light on the 

 question of tissue differentiation. For this question metazoan 

 material must be used. No successful nuclear transfers have been 

 reported for other invertebrates although Danielli (1955) stated 

 that preliminary attempts have been made on sea urchin and 

 ascidian eggs. All remaining studies reported to date have dealt 

 with amphibian eggs and, of these, by far the most extensive and 

 successful experiments have been carried out by Briggs and 

 King working with the frog, Rana pipiens (Briggs and King, 

 1952, 1953, 1955; King and Briggs, 1953, 1954a,b, 1955). 



In brief, then- transplantation technique involved activation of 

 eggs with a clean glass needle; this is sufficient to cause the egg 

 to rotate and initiate the second maturation division. A few 

 abortive furrows may appear, but no true cleavage or blastulation 

 takes place (Briggs, Green, and King, 1951). Guyer (1907) 

 showed that, in the absence of tissue fluid contaminants, pricking 

 alone is not sufficient to stimulate parthenogenetic development 

 in the frog egg. The egg nucleus was removed by the method of 

 Porter ( 1939 ) in which a needle is inserted under the egg nucleus 

 and drawn upward, causing an exovate of egg cytoplasm which, 



