H. E. LEHMAN 221 



case of mid-gut nuclei from late gastrulae of R. pipiens (King 

 and Briggs, 1955), and this example stands as the only positive 

 evidence of nuclear differentiation yet obtained from nuclear 

 injection experiments. As such, it represents a real advance in the 

 understanding of this problem. However, even this example does 

 not express its nuclear specialization in cytoplasmic responses 

 that are limited to the tissue-specific determination of the in- 

 jected mid-gut donor cells. It is therefore apparent that nuclear 

 and cytoplasmic determinations do not proceed at the same pace 

 since it is well known from the work of Holtfreter ( 1938a,b ) that 

 as early as the late blastula stage the endoderm consists of an 

 almost rigid mosaic of organ-specific irreversibly determined cells. 

 The nucleus of a determined cell may well remain in a labile 

 state of differentiation and/or retain a wider range of histogenic 

 potentialities than the cytoplasm surrounding it. An answer to 

 questions concerning the possibility of tissue-specific nuclear 

 specialization and labile versus irreversible nuclear determination 

 must await additional information from transplantations of more 

 advanced nuclei, preferably from cells showing histological or 

 biochemical differentiation as well as detemiination. It is prob- 

 ably not until quantitative and qualitative differences in the 

 macromolecular composition of the embryonic tissues is distinct 

 that one should expect to find nuclei restricted to tissue-specific 

 functions. 



Conclusion 



Evidence from biochemical, cytological, and nuclear transplan- 

 tation studies has been reviewed which indicates that in certain 

 instances at least tissue-specific differences can be demonstrated 

 in the nuclei of histologically determined cells within embryos 

 and adult organisms. These examples stand at present as excep- 

 tions to the generally held concept of quantitative and qualitative 

 nuclear uniformity in somatic cells, a concept which to a large 

 measure is based on evidence obtained from the demonstration 

 of nuclear totipotency in early cleavage cells and regenerating 

 tissue. Nuclear transplantation studies suggest that embryonic 

 nuclei are totipotent until gastrulation and may retain the capacity 



