No. 627] 



ADAPTATION 



349 



and that the result is often a very imperfect reconstruc- 

 tion of the whole, lends support to the view that the cells 

 of the injured embryo "feel their way"— so to speak- 

 back into a condition of mutual equilibrium. In some 

 cases this equilibrium appears to be of a simple physical 

 sort, as for instance, that which is brought about by the 

 folding together of the edges of a blastula fragment so 

 as to reconstruct the spherical form. But in most cases 

 the factors are doubtless vastly more complex. 



Once the reconstruction of the normal embryonic form 

 is attained, the difficulties in understanding the further 

 stages of ontogeny are no greater than we meet with in 

 the case of an uninjured embryo— that is, unless we are 

 encumbered by a preformation theory of development. 



2. As regards the second point above raised, there is 

 theoretically no greater difficulty in understanding how 

 one tenth of an organism may restore the remaining nine 

 tenths than in understanding how the nine tenths may 

 restore the one tenth. As a matter of fact, in dealing 

 with certain organisms, the size or shape of the piece, or 

 the region of the body from which it is taken count for 

 little in the outcome. But they do count for something, 

 and that something is significant. It has been found in 

 some cases, for example, that there are lower limits to 

 the side of the pieces which may carry out development 

 or regeneration. And in other cases, the position of the 

 plane of section may deteiiiiine whether a useful strnc- 

 ture is formed or one which is wholly useless. 



But whether or not the size or shape of the fragment 

 count for anything in the reparation of a given organism, 

 we find that the species from which it is taken counts for 

 everything. There must, therefore, be something that 

 is common to all detached portions of an organism which 

 are capable of reconstructing the same whole. The por- 

 tion in question may be an asexual spore or a fertilized 

 egg, or it may be an isolated blastomere or other arti- 

 ficially detached fragment of either an embryo or adult 

 organism. What is this greatest common divisor? Is 

 it a unit of structure or is it a chemical substance? 



