220 NUCLEAR TRANSPLANTATION 



creases with the age and differentiation of the donor nuclei and 

 therefore parallels the evidence interpreted as an indication of 

 nuclear differentiation. 



(Briggs & King, 1952) 

 (King & Briggs, 1954a) 



(King & Briggs, 1954a) 



Note: No data on ploidy were included with the most recent and convincing 

 evidence of nuclear differentiation in late gastrula mid-gut cells reported by 

 Briggs and King (1955). 



It is well known that, so long as complete sets of chromosomes 

 are duplicated, a moderate increase in ploidy is compatible with 

 normal development (see review by Fankhauser, 1955). The 

 results of Briggs and King have shown that polyploidy in "nuclear 

 transplant" embryos bear this out in that some of their best and 

 most long-lived cases were polyploids. However, when chromo- 

 some duplication involves only part of the chromosome comple- 

 ment (aneuploidy) viability is greatly impaired and differentia- 

 tion is usually abnormal (see reviews by Fankhauser, 1945, 1952, 

 1955). The latter fact should be considered along with the evi- 

 dence from chromosome counts on "nuclear transplant embryos" 

 of Triton which revealed a high incidence of hyperdiploids and 

 diploid-hyperdiploid mosaics (Lehman, 1955). This alone could 

 be a significant factor contributing to the generally poor develop- 

 ment that was obtained with Triton eggs. The technique for deter- 

 mining ploidy used by Briggs and King ( namely, nuclear size and 

 nucleolar number) cannot yield precise information concerning 

 degrees of aneuploidy. 



Considerable caution should be exercised in deducing positive 

 evidence for nuclear differentiation from arrested or atypical 

 "nuclear transplant embryos" unless there is a cytological basis 

 for demonstrating euploidy and/or a consistent syndrome in the 

 anomaly obtained with a given type of detennined donor cell 

 nucleus. The latter condition has been satisfactorily met in the 



