H. E. LEHMAN 213 



exists that recently determined neural cells of the frog may 

 possess nuclei in a labile state of differentiation. 



When late gastrula "chorda-mesoderm" nuclei were injected 

 into enucleated eggs, a few cases developed into entirely normal 

 larvae, one of which was carried to metamorphosis (King and 

 Briggs, 1954a). However, in a great majority of the cases, devel- 

 opment was arrested by the late neurula stage ( King and Briggs, 

 1955). Histological sections of the abnormal postneurulae re- 

 vealed that brain, spinal cord, notochord, gut, pronephric tubules, 

 heart, blood vessels, somites, and sense organs developed (King 

 and Briggs, 1954a ) . Neural differentiation was generally reduced 

 or entirely absent. It was suggested that the donor "chorda-meso- 

 derm" nucleus, although capable in some instances of giving rise 

 to total nomial development, might be partially deficient in its 

 control of the ectodermal response to neural induction (King 

 and Briggs, 1955). However, on the basis of the best cases ob- 

 tained, it would appear that these nuclei, at most, are determined 

 in an entirely labile manner at the late gastrula stage. 



The most decisive evidence of nuclear specialization during 

 development was obtained when determined mid-gut endoderm 

 nuclei of late gastrulae were injected into enucleated eggs (King 

 and Briggs, 1955). In 40% of the attempts, complete blastulae 

 were obtained. All failed to develop normally in gastrula and 

 postgastrula stages. The most advanced experimental embryos 

 exhibited a consistent combination of anomalies not observed in 

 other embryos. These included epidermal disorganization in- 

 volving localized deficiencies and unequal thickness, poor neural 

 development with cytological evidence of nuclear degeneration, 

 well-developed notochord, differentiated somites of abnormal 

 form, and a gut "developed as well as the general condition of 

 the embryo allows" (King and Briggs, 1955, p. 324). Endodermal 

 cytoplasm alone did not produce this result when injected into 

 normally fertilized eggs. On the basis of these data, King and 

 Briggs seemed justified in revising their earlier and more con- 

 servative interpretations (King and Briggs, 1954a; Briggs and 

 King, 1955) and in concluding that "it now appears definite that 



