208 NUCLEAR TRANSPLANTATION 



out endangering the life of the cell. He points out that this hy- 

 pothesis resembles that of Weismann but differs "in that the ex- 

 periments which discredited the Weismannian point of view 

 (nuclear differentiation at the cleavages) are not relevant to it. 

 It is closer to the hypothesis of changes in the centers of ac- 

 tivity of the genes during differentiation of the tissues " which 

 was earlier suggested by Morgan (1934) as a possible mechanism 

 whereby determined cytoplasm selectively affects genes which in 

 turn maintain the differentiation of the cytoplasm in a reciprocal 

 interaction between these two basic cell components. Until addi- 

 tional information is available concerning the dynamic properties 

 of heterochromatin in interphase nuclei, it is not possible to assay 

 its significance in cell function and differentiation. 



The essential fact from the foregoing summary of visible dif- 

 ferences in somatic cell nuclei is that there are now numerous 

 "exceptions" to the concept of quantitative and qualitative nu- 

 clear uniformity in somatic tissues. As Huskins (1947) has in- 

 timated, additional studies, unbiased by the preconception of 

 nuclear uniformity, would undoubtedly add materially to the 

 growing body of data that points toward nuclear specialization as 

 a general, rather than an exceptional, aspect of histogenesis. 

 In collecting these data, however, the fact still remains that the 

 cytological evidence of nuclear differentiation in cells of embryos 

 most suitable for experimental study are meager. It is hoped that 

 additional cytogenetic and cytochemical studies will concentrate 

 attention on species favorable for developmental analysis so that 

 the varied techniques from these disciplines can jointly be 

 brought to bear on this morphogenetic problem. One relatively 

 new embryological method, namely, nuclear transplantation, has 

 recently been applied to the question of nuclear differentiation. 

 With more extensive application, it should constitute an impor- 

 tant supplement to the cytological and biochemical information 

 now available on this subject. 



Technique and Results of Nuclear Transplantation 



The simplest and most direct manner in which information 

 could be obtained concerning irreversible nuclear determination 



