216 NUCLEAR TRANSPLANTATION 



cannot be answered in the light of present knowledge. It will be 

 particularly interesting in these connections to learn of the results 

 being obtained with improved techniques for transplantation of 

 postgastrula nuclei which King and Briggs ( 1955 ) indicate are 

 in progress. To complete the picture, it would also be desirable to 

 learn the developmental capacities of blastula and young gastrula 

 endoderm nuclei to discover when nuclear specialization in these 

 cells is first demonstrable. 



Blastula and gastrula nuclei from R. pipiens 9 xR. catesheiana 

 $ diploid hybrids and from R. pipiens ( 9 ) x R. catesheiana $ 

 andromerogonic haploid hybrids have also been transplanted into 

 enucleated R. pipiens eggs (Briggs and King, 1952; King and 

 Briggs, 1953). These experiments were designed to analyze the 

 role of the nucleus in the well-known lethality which occurs dur- 

 ing gastrulation in R. pipiens 9 x R. catesheiana S crosses (see 

 review by J. A. Moore, 1955). Since this subject is aside the major 

 line of interest here and is treated in a separate section of this 

 volume (see J. R. Gregg), it will suffice to say that potentially 

 lethal hybrid nuclei from undetermined blastula and gastrula 

 cells could be transferred into enucleated eggs and would par- 

 ticipate in normal cleavage and blastulation, but they would not 

 permit complete gastrulation. This was true provided that the 

 donor nucleus had not advanced to the stage when irreversible 

 degenerative changes take place which anticipate lethality in 

 the donor hybrids. The hybrid donor nuclei therefore gave evi- 

 dence of being able to survive long after they would have died 

 had they remained in the donor embryos. These results also show 

 that lathality in this cross is not due to autonomous nuclear 

 change, but it is the result of failure on the part of R. catesheiana 

 nuclear material and R. pipiens cytoplasm to cooperate in postgas- 

 trula development. Related to this study is a brief report (Briggs 

 and King, 1955) of transplanting nuclei from blastulae of the 

 anuran, Triturus pyrrhogaster, into enucleated R. pipiens eggs. 

 Partial cleavage and blastulae were obtained in the best cases. 

 Cytological study showed that the pyrrhogaster cell probably 

 supplied a centrosome which was capable of quite normal func- 

 tion as a cleavage center in pipiens cytoplasm. The foreign nu- 



