244 REGENERATION 



According to Roux, the first division of the frog's egg divides the 

 material of the right half of the embryo from that of the left ; the 

 second division separates the material of the anterior half from that 

 of the posterior half. Roux limited, to a certain extent, his hypothesis 

 to these two divisions of the frog's egg, and stated further that it is 

 not improbable that during the later stages of development there may 

 take place an interaction of the parts on each other, and this inter- 

 action would be another factor in the development. Weismann has 

 adopted Roux's hypothesis, and has extended it to all organisms, and 

 to most of the divisions of the developing egg, at least to all those 

 divisions in which the qualities of the layers, tissues, organs, etc., are 

 separated. On this slight basis he has constructed his theory of 

 development and of regeneration It is important, therefore, to 

 examine critically the evidence furnished by experimental embryology 

 for or against this hypothesis of a qualitative division of the egg dur- 

 ing the cleavage period. 



The development of a half embryo from one of the first two 

 blastomeres of the frog's egg, in Roux's experiment, seemed to sup- 

 port Roux's hypothesis, but it was not long before it was seen that 

 the presence of the other blastomere vitiated the evidence to such 

 an extent as to render it worthless, so far as this hypothesis is 

 concerned. Then followed the experiments with the isolated blasto- 

 meres of the sea-urchin, amphioxus, jelly-fish, teleost, ascidian, triton, 

 etc., in which each blastomere, when completely separated, gives rise 

 to a whole embryo. From these experiments Driesch and Hertwig 

 drew the opposite conclusion, namely, that during the cleavage there 

 is a quantitative division of the egg into blastomeres that are equiva- 

 lent, or at least totipotent. Roux attempted to meet the results of 

 these experiments in two ways. He pointed out that in several of 

 these cases the isolated blastomere divides as a half or as a fourth 

 of the egg, and that in the sea-urchin this leads to the formation of an 

 open half-blastula. In the second place, Roux brought more to the 

 front his subsidiary hypothesis of the reserve germ plasm. He sup- 

 posed that along with the early qualitative division of the nucleus, 

 by means of which each part receives its particular chromatic sub- 

 stance, there is also a quantitative division of a sort of reserve germ 

 plasm contained in the nucleus. Each cell may receive also a part 

 of this material, and hence each cell may contain the potentialities 

 of the whole egg. This reserve plasm may be awakened by any 

 change that alters the normal development, as, for instance, when 

 the blastomeres are separated. It may take some time for this 

 reserve stuff to wake up, as shown by the half-development of the 

 sea-urchin's egg that goes on for some time after the separation 

 of the blastomeres. This hypothesis cannot be objected to on purely 



