222 NUCLEAR TRANSPLANTATION 



for participating in total development even after the cells contain- 

 ing them are histologically determined. However, with advancing 

 age and progressive differentiation there is an indication that 

 embryonic nuclei become restricted in their developmental ca- 

 pacities. These data would indicate that nuclear determination 

 and differentiation, if they do indeed regularly occur during 

 histogenesis, do not take place concurrently with the initial de- 

 termination of the cell as a whole. One is led to suspect that 

 determination is gradually imposed on the nucleus by its sur- 

 rounding cytoplasm. There is no information available at present 

 to clarify the question of labile versus iireversible nuclear differ- 

 entiation; nor is it possible in the light of current knowledge to 

 decide whether or not cytoplasmic differentiation accompanied 

 by nuclear differentiation is a general or an exceptional aspect of 

 histogenesis. 



In concluding this summary of evidences for nuclear differen- 

 tiation it is safe to say that even the most guarded and conserva- 

 tive evaluation must admit the possibility of progressive nuclear 

 differentiation in development. Stern ( 1955 ) has pointed out that 

 this hypothesis is not required to account for local developmental 

 differences, but the possibiHty is compatible with the general 

 array of facts provided by genetics, physiology, and embryology. 

 Additional examples of restriction in nuclear function correlated 

 with cytoplasmic differentiation are needed from older tissues 

 and other species before it will be possible to say with any degree 

 of certainty that nuclear differentiation does indeed characterize 

 histogenesis generally. If this primary question is answered af- 

 firmatively by new data, it will be appropriate to consider sec- 

 ondary questions beyond the scope of this paper relative to the 

 manner in which detemiined cytoplasm "induces" nuclear change, 

 as well as the nature of the reciprocal influence of the nucleus 

 in maintaining cytoplasmic function (for theories already ad- 

 vanced relating to these questions, see reviews on: auto-anti- 

 bodies, Tyler, 1946; Weiss, 1950, 1955; Ebert, 1954; Woerdeman, 

 1955; plusmagenes, Mather, 1948; Schultz, 1950, Sonneborn, 

 1951; Stern, 1955; enzyme adaptation, Monod, 1947; Spiegehnan, 

 1948; Stanier, 1954). With regard to these latter questions one of 



