202 NUCLEAR TRANSPLANTATION 



differentiated cells also retain a large measure of their totipotency 

 (see reviews by Schotte, 1939; Child, 1941; Needham, 1952). In 

 the light of this pressure of evidence it is not surprising that the 

 question of nuclear differentiation has received little attention in 

 most experimental studies of development. 



It should be realized that, although the prevailing consensus 

 accepts the concept of nuclear uniformity in somatic tissues, the 

 possibility of nuclear differentiation has never been conclusively 

 eliminated with regard to the cells of late embryonic or adult tis- 

 sues. During recent years the need for reconsideration of this 

 question has been expressed with increasing frequency (e.g., 

 Huskins, 1947, 1952; Huskins and Steinitz, 1948a,b; Schultz, 

 1952; Stern, 1955). In this connection it is noteworthy that the 

 embryonic evidence for nuclear uniformity is primarily based 

 upon studies of nuclei in early cleavage stages only, the most 

 striking demonstration being that supplied by the constriction 

 experiments on eggs of Triton in which total development re- 

 sulted in fragments that received only one of the 2-, 4-, or 8-cell 

 stage blastomere nuclei (Spemann, 1914, 1928; Fankhauser, 

 1925, 1930). When single nuclei from 16- and 32-cell stages en- 

 tered non-nucleated egg fragments, partial development of the 

 fragment resulted. This failure to complete development has gen- 

 erally been attributed to irreversible cytoplasmic changes that 

 resulted from prolonged isolation from a nucleus. Although this 

 may be true, these observations also admit the seldom considered 

 alternative, namely, loss of nuclear totipotency which is nuclear 

 specialization. 



These and related experiments (see review by Spemann, 1938) 

 have demonstrated conclusively the equivalence of early cleavage 

 nuclei, but they throw no light on the nuclear character of older 

 cells. Predictions from these data that all somatic nuclei remain 

 equivalent during development are particularly insecure in view 

 of the extensive body of information which indicates that for the 

 most part the cytoplasm appears to be almost completely autono- 

 mous during cleavage and blastulation and very few nucleus- 

 dependent functions have been demonstrated during this early 

 phase of development. For example, enucleated eggs and egg 



