REGENERATION I 43 



half of the egg. Roux certainly states that • post-generation ■■ 

 does not occur in the same manner as does the normal develop- 

 ment of the two primary halves, — that is to say, the germinal 

 layers are not formed independently in each ; but the processes 

 which take place in the interior of the ovum can only be followed 

 out by means of sections, the preparation of which necessitates 

 the killing of the embryo. 



In such experiments, moreover, no two cases are alike, and it 

 would be necessary to examine a very large amount of material 

 before stating with any degree of certainty that the egg which 

 has been cut into sections, and that in which the development 

 and post-generation have been followed out, have a precisely 

 similar internal structure. 



Roux observed a 're-animation' of three different kinds in the 

 halves of the eggs operated upon, one of which consisted in a 

 growth of the cells in the external layer of the living half around 

 the dead half. In this instance, however, post-generation did 

 not result : it only occurred in certain, but not all. of those cases 

 mentioned above in which nuclei passed from the living half 

 into the part which had been operated upon, and in which only 

 slight pathological changes had occurred in the yolk. It is there- 

 fore natural to suppose that post-generation only occurred when 

 the injury was a slight one, and when some nuclear matter 

 remained and subsequently caused a formation of cells. This, 

 however, does not imply that living 'nuclei' did not penetrate 

 into the injured half of the egg; the segmentation-cells, even in 

 normal development, have to undergo an enormous increase, 

 and it is therefore not surprising that after the opposition to 

 growth has been removed by the operation on the other half of 

 the egg, they should increase at the expense of the latter. In 

 those cases in which the other half of the embryo was subse- 

 quently completed, this completion must have resulted from a 

 kind of infection of the cell, of such a nature that mere contact 

 with ectoderm or mesoderm cells, for example, caused the undif- 

 ferentiated cells of the injured half of the egg to become corre- 

 spondingly differeniiated into ectoderm and mesoderm cells. 

 But I could only accept such a revolutionary hypotliesis as this 

 if it could be proved by incontestable facts. 



Roux himself has, however, only looked upon his contribu- 

 tions to this subject as 'a first instalment of a large work,' and 

 has led us to expect a continuation of his experiments. But as 



