214 NUCLEAR TRANSPLANTATION 



nuclei undergo certain changes during differentiation . . . ex- 

 periments not reported here reveal that at later developmental 

 stages there is a loss of the capacity of these nuclei to enter into 

 cleavage of egg cytoplasm. All together, this suggests a progres- 

 sive specialization of nuclear function during cell differentiation" 

 (King and Briggs, 1955, p. 324). 



The writer is aware of only one attempt to repeat the studies of 

 Briggs and King on eggs of R. pipiens. This consists of an unpub- 

 lished study by Dr. C. L. Markert who has generously given 

 permission to have his data summarized here. The enucleation 

 and transplantation technique was essentially similar to that used 

 by Briggs and King and the results outlined in a written com- 

 munication are of sufficient interest and corroborative value to be 

 quoted in full: 



"In the initial series of experiments [performed in collaboration 

 with Arthur Freedman] a total of 256 nuclei were transplanted. 

 Of the eggs receiving these transplanted nuclei, 117 showed no 

 signs of development; 34 underwent abortive cleavage; 40 showed 

 extensive cleavage; 32 developed to blastula; 20 gastrulated (gen- 

 erally abnormal); 9 became well developed neurula, and 4 de- 

 veloped into larvae. One of the larvae was tetraploid. I cite these 

 data as illustrative of the discouragingly low level of success that 

 we achieved. Later I resumed this work with a higher percentage 

 of success but the general picture remained essentially un- 

 changed. A variety of cell types was used as a source of nuclei in 

 these experiments. Nearly all of the successful transplants in- 

 volved nuclei from cells of blastula or early gastrula embryos. 

 One of the neurulae — an abnormal one — developed after trans- 

 plantation of a nucleus from an endoderm cell taken from the 

 floor of the archenteron in the late neurula stage of development. 

 This was the most advanced stage of development induced by 

 nuclei taken from post-gastrulae. However, cleavage was induced 

 occasionally even with nuclei from larvae old enough to show 

 muscular responses" (C. L. Markert, unpulDlished experiments). 



These results are in agreement with those of Briggs and King 

 which indicate that normal development is less frequently ob- 

 tained as the age of the donor nucleus increases. The concluding 



