CLEAVAGE AND DIFFERENTIATION 9I 



happens at the 8-cell stage, it might be expected that this separa- 

 tion of different portions of the primary egg-axis would mean an 

 unequal distribution of potencies, and, as will be seen later, this 

 expectation is in fact realised. 



The case of the frog is in principle similar to that of the newt. 

 But the experiments conducted from time to time on the frog have 

 suffered so much from unforeseen complications, that the con- 

 clusions drawn from them w^ere for a long time misleading. The 

 chief difficulty arises from the fact that the eggs of Anura have 

 long defied attempts to secure the constriction and separation of 

 blastomeres. Experimental technique has therefore been largely 



Fig. 40 



One embryo from two eggs. Left, two Triton eggs in the 2-cell stage are laid 

 across each other, so that their blastomeres alternate. Centre, each blastomere 

 has divided once. Right, a giant neurula resulting from such a fusion. (After 

 Seidel, from Morgan, Experimental Embryology, Columbia University Press, 

 1927, modified.) 



restricted to injuring one of the blastomeres : this is usually accom- 

 plished with a hot needle. The result of such an experiment at the 

 2-cell stage is that the uninjured blastomere develops into a half- 

 embryo, and does not produce much more than it would have done 

 if its sister-blastomere had developed normally alongside it, for it 

 is a condition of the experiment that the injured blastomere re- 

 mains attached to the uninjured one.^ (For the present purpose, 

 the subsequent attempt of the half-embryo to complete itself by 

 " post-generation "'-^ may be passed over here as irrelevant (fig. 41).) 



1 Roux, 1888. 



^ In some of the cases originally described by Roux, the half-embryo obtained 

 by injuring one blastomere with a hot needhe appeared to be subsequently con- 

 verted into a whole embryo, by the utilisation of the materials of the injured 

 blastomere. To this restorative process, the name "post-generation" was given. 

 It was imagined that the reorganisation of the injured blastomere was brought 

 about either by belated cleavage of its nucleus, or by invasion of cells from the 

 uninjured half, or by overgrowth of the injured half by layers of tissue from the 



