364 PRINCIPLES OF EMBRYOLOGY 



There are therefore some grounds for thinking that all genes may be 

 active, and producing effects of some kind, in all the cells of the body, even 

 in those in which, owing to the canalisation of developmental processes, 

 the influences of the mutant alleles do not suffice to produce any diver- 

 gence from the normal. It must be emphasised that this suggestion remains 

 no more than a hypothesis, and that it is quite possible that in some tissues 

 certain genes become completely and irreversibly inactivated or lost : the 

 point will not be fmally decided until it is possible to transplant nuclei 

 from differentiated cells into cytoplasms of an earher and not-yet-deter- 

 mined stage and to discover whether such nuclei still retain the full range 

 of developmental potentialities. Briggs and King (1952, 1953) were the 

 first to acliieve any important success in this. Frogs' eggs were partheno- 

 genetically activated by pricking with a glass needle, and the egg nucleus 

 removed (fortunately it can be located with some certainty in this form). 

 A nucleus with a little associated cytoplasm from a cell of a later embryo 

 was then injected into the enucleated egg. Cleavage followed in a fair 

 number of cases. With nuclei taken from morula, blastula or early gastrula 

 stages, complete development of the host egg into a fully differentiated 

 larva sometimes occurred. This proves that, as might be expected, no 

 irreversible change has occurred to the nucleus during these early stages, 

 before the onset of determination, let alone of cellular differentiation. 



More recently, similar results have been reported with nuclei from the 

 determined but not yet differentiated tissues of the late gastrula. Wadding- 

 ton and Pantelouris (1953), working with newts' eggs, found that such 

 nuclei were just as good as earlier ones for enabling cleavage to occur, 

 but in their material the host eggs always stopped at the beginning of 

 gastrulation, whatever the age of the transplanted nucleus; this was 

 probably due to the inadequacy of the technique rather than of the nuclei. 

 King and Briggs (1954), however, have succeeded in transplanting such 

 nuclei in frogs, and have shovvm that they can control the development of 

 completely normal tadpoles. Thus determination does not involve any 

 irreversible loss of gene function ; it still remains uncertain how far this 

 is also true of differentiation. 



4. The mechanism of gene activation and inhibition 



One would, of course, like to understand the mechanism by wliich the 

 cytoplasm influences the nucleus and stimulates or inhibits the activity 

 of various genes. There are several possibilities which must be envisaged. 

 In the first place, so long as division continues m a cell-lineage, the genes 

 in the nucleus must be synthesising duplicates of themselves so as to pro- 

 vide the increasing number of chromosomes ; and at the same time and 



