NUCLEAR TRANSPLANTATION, 

 A TOOL FOR THE STUDY OF 

 NUCLEAR DIFFERENTIATION 



H. E. LEHMAN: department of zoology, 



UNIVERSITY OF NORTH CAROLINA, 

 CHAPEL HILL, NORTH CAROLINA 



The problems of embryonic determination and differentiation 

 are concerned primarily with the mechanisms whereby a limited 

 part of the total capacity of the genome is selected for phenotypic 

 expression in different somatic cells of the developing organism. 

 A substantial body of information derived from nmnerous studies 

 on isolated blastomeres, induction systems, physiological gradi- 

 ents, etc. indicates that the selective mechanism in determination 

 is extranuclear in origin and is dependent primarily upon local 

 cytoplasmic differences. This view is in agreement with the con- 

 ventional opinion which accepts the concept of "constancy of 

 chromosome number" of Hertwig (1916) and assumes that nuclei 

 are quantitatively and qualitatively equivalent throughout the 

 somatic cells of the body. The theory of Weismann ( 1885, 1892 ) 

 which postulated fractional allocation of nuclear materials to 

 different blastomeres as the primary mechanism for somatic dif- 

 ferentiation has generally been abandoned since the equivalence 

 of blastomere nuclei has been repeatedly demonstrated on repre- 

 sentative eggs of most major phyla. Normal development can still 

 take place after the cleavage nuclei are shifted to various "un- 

 natural" positions by compression, centrifugation, or constriction 

 of the egg (see reviews by Wilson, 1925; Morgan, 1927; Spe- 

 mann, 1938). A host of regeneration studies on vertebrates, in- 

 vertebrates, and plants have supplied evidence which, although 

 inconclusive, has been interpreted as an indication that nuclei of 



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