H. E. LEHMAN 215 



statement of Markert is of particular interest in giving promise of 

 success in transplanting nuclei from histologically differentiated 

 cells with further refinement of the operative technique. 



In evaluating the results of late gastrular nuclear transplants, 

 it can be said with certainty that irreversible nuclear specializa- 

 tion does not take place at a uniform rate throughout the embryo. 

 Apparently none has occurred by this time in prospective ecto- 

 derm derivatives; chorda-mesoderm nuclei may illustrate an inter- 

 mediate condition, whereas endoderm nuclei are clearly special- 

 ized. The data from "mid-gut embryos" probably should be inter- 

 preted as evidence of quantitative, rather than qualitative, modifi- 

 cation in nuclear function since, in the most advanced cases at least, 

 all tissue types were represented to a greater or lesser degree. Even 

 ectoderm, although poorly developed, showed a slight capacity 

 for neural development. An alternative interpretation admits the 

 possibility of tissue-specific (i.e., qualitative) nuclear difl^erentia- 

 tion. For example, current evidence does not peiTnit one to de- 

 cide whether the limited neural capacity in "mid-gut embryos" 

 stemmed from the transplanted nucleus (as is assumed above), 

 or is the expression of a residual effect of the egg nucleus dating 

 from the period of oogenesis. The latter possibility should not be 

 overlooked, since the well-known studies of Hammerling ( 1934, 

 1953) on regeneration in Acetabiilaria, and of Hadorn (1936) 

 on andromerogonic Triton hybrids [T. palmatus ( 9 ) x T. crista- 

 tiis S ] have shown that species-specific characters of nuclear 

 origin can be maintained in the cytoplasm of enucleated cells 

 and in the presence of a foreign nucleus for extended periods of 

 time (several months in Acetabiilaria; until postmetamorphosis 

 in Triton hybrid merogon tissues when transplanted to diploid 

 hosts). This second possibility gains some support from the cyto- 

 logical condition of the epidermal nuclei in "mid-gut embryos" 

 which showed abnormal degenerati\ e structure that suggests an 

 incompatibility between irreversibly determined cytoplasm and 

 nuclei. This question, along with those relating to the influence of 

 donor cytoplasm injected with the transplanted nuclei, and the 

 possible effects of nuclear damage in operative manipulation (in 

 particular the trypsin-versene method of isolating donor cells) 



