142 THE GERM-PLASM 



injured fellow blastomere. This doubt seems, however, to be 

 hardly justified, as such an isolation was not effected in Chabry's 

 experiments on the ascidian ovum, but nevertheless the develop- 

 ment into a complete animal ensued just as in the case of the 

 egg of the sea-urchin. 



Although the half of a frog's egg develops into half an embryo 

 only in the first place, the latter may subsequently become com- 

 pleted by a very peculiar regenerative process, which was first 

 observed by Roux in 'half and 'three-quarter embryos,'' and 

 which he designated as 'post-generation.' 



Roux observed that a segmentation-cell of a frog's egg may be 

 ' re-animated ' after it has been deprived of its capacity for devel- 

 opment. A considerable number of nuclei pass into the vitellus 

 of the injured part from the normally developed half of the egg, 

 and there increase and give rise to cells. ' The post-generative 

 formation of the germinal layers takes place from the cell-material 

 subsequently formed, while the process of differentiation contin- 

 ues to advance in the quiescent cell-material.' Roux thought he 

 observed that a complete restoration of the embryo may take 

 place in this manner, so that it can continue to live ; and, in 

 fact, he actually succeeded in keeping such an embryo alive for 

 some time. 



Considerable attention has naturally been drawn to these 

 observations, which are certainly of the greatest interest ; but 

 I doubt whether in their present state they are .sufficiently com- 

 plete to form the basis of fundamental theoretical conclusions. 

 With all respect for Roux's accuracy of observation and skill 

 in research, I cannot help thinking that the half embryos which 

 were subsequently ' post-generated ' to entire animals, were pos- 

 sibly those in which the thrust with the hot needle had not 

 affected the nucleus of the segmentation-cell. 



In any case, it was only possible to observe the actual effect 

 of the operation and its result on the whole series of processes 

 which followed, and which led to the restoration of individuals 

 otiier than those which ultimately became complete. To pierce 

 a segmentation-cell with a hot needle must be a tolerably rough 

 operation, and something different may be destroyed each time 

 it is performed : not only the nuclear matter as a whole, but 

 also the individual idanis, might possibly remain uninjured. 

 The idants, again, might subsequently increase to the normal 

 number by doubling, and so bring about the development of the 



