212 NUCLEAR TRANSPLANTATION 



irreversibly determined at the time of transplantation since they 

 were capable of replacing the zygote nucleus and of participating 

 in total nomial development. 



With the foregoing demonstration of the feasibility of this op- 

 erative technique, Briggs and King have proceeded to improve 

 the method and to test the possibility of nuclear differentiation 

 in determined embryonic cells by using gastrulae, neurulae, and 

 tail-bud embryos as sources of donor nuclei. In general it can 

 be said that the older the donor nucleus, the lower is the percent- 

 age of successful cases which show cleavage, gastrulation, and 

 postgastrula development. As yet, no cases involving neurula or 

 tail-bud nuclear transplants have been reported which gastru- 

 lated normally, although a small number have undergone normal 

 cleavage (King and Briggs, 1954a). This may be due to technical 

 difficulties encountered in handling the smaller cells. It is also 

 possible that this developmental failure may be associated with 

 a restriction in the capacity of older nuclei to participate in total 

 development. The strongest evidence in support of the latter pos- 

 sibility is supplied by transplantation of nuclei from determined 

 cells of late gastrulae (Briggs and King, 1953; King and Briggs, 

 1954a, and, particularly, 1955). 



The results from transplanting nuclei from recently induced 

 neural ectoderm from late gastrulae into enucleated eggs showed 

 that cases reaching the blastula stage could develop into entirely 

 normal tadpoles (King and Briggs, 1954a). Although the donor 

 nucleus came from a cell determined to form neural tissue, it is 

 apparent that if any tissue-specific nuclear differentiation existed 

 at the time of transfer, it was completely reversible. Briggs and 

 King ( 1955 ) pointed out that these results should be viewed in 

 context with the observations of Grobstein ( 1952 ) and Grobstein 

 and Zwilling ( 1953 ) who have found that, although large masses 

 of explanted mouse or chick neural plate will form nervous tissue, 

 small fragments fail to differentiate. This indicates that the neural 

 plate may be irreversibly determined as a whole while its com- 

 ponent cells remain labile and incapable of self-differentiation. 

 If the same situation were to obtain in Amphibia, the possibility 



