248 RE GENERA TION 



the origin of the bilateral symmetry. In many eggs there is no evi- 

 dence of a bilateral distribution of the material, although in some few 

 cases there may be, so far as the form is concerned, a plane of bilat- 

 eral symmetry. But even if it is supposed to be present in all eggs, 

 and to coincide with the first plane of cleavage (or with any other 

 cleavage plane), we still could not explain the bilateral symmetry of 

 the one-half and one-fourth whole embryos that come from the corre- 

 sponding isolated blastomeres. If a preexisting bilateral plane exists 

 in the egg, it must be reestablished in some way in the isolated blasto- 

 mere and in pieces of the blastula wall. In the latter case this could 

 scarcely be brought about by a redistribution of the gross contents 

 of the piece, since the presence of cell walls would interfere with such 

 a process. 



This analysis shows, I think, that the transformation of a piece 

 into a new whole really involves a change in the fundamental struc- 

 ture itself. There cannot be much doubt that both the polarity and 

 the bilaterality of the egg, or of a piece of the egg, belong fundament- 

 ally to the same class of phenomena, and we are forced to the sup- 

 position that they are inherent peculiarities of the living substance. 

 Driesch thought at one time that it is only necessary to suppose that 

 the protoplasm, and every part of it, possesses a primary polarity, 

 and that some inequality in the material might determine the plane 

 of bilaterality; but later he thought it necessary to assume also the 

 presence of a bilateral structure in the protoplasm, and in all parts of 

 it. This assumption of every part having a polar and a bilateral struc- 

 ture, and the polarity and bilaterality of the whole being the sum 

 total of those of all its parts, is, I think, insufficient to meet the situa- 

 tion. If, for example, the first plane of cleavage coincides with the 

 median plane of the body, the right blastomere has a structure that 

 leads to the formation of the right side of the body, and similarly 

 for the left blastomere. If the two blastomeres are separated, 

 and each gives rise to a whole embryo with a new plane of bilateral 

 symmetry, we must suppose that a new bilaterality has been 

 produced. It does not make the problem any simpler to assume, as 

 Driesch has done, that this is brought about by the elements re- 

 arranging themselves bilaterally on each side of a new plane that 

 passes through the middle of the isolated blastomere, for what we 

 need to have explained is what determines the new median plane. 

 It seems to me that the problem is not any simpler, if we assume the 

 polarity and bilaterality to be the property of a large number of ele- 

 ments, as Driesch has done, than if we assume at once the polarity 

 and bilaterality as characteristic of the whole egg. The difficulty of 

 understanding how a new bilaterality can be induced in a piece of the 

 whole is as great on the one assumption as on the other. Not only 



