NATURE AND CAUSES OF DIFFERENTIATION 413 



Stages as well, as Koiliker insisted in opposition to Weismann as 

 early as 1886, and as has been urged by many subsequent writers. 

 The strongest evidence in this direction is afforded by the facts of 

 Regeneration; and many cases are known — for instance, among the 

 hydroids and the plants — in which even a small fragment of the 

 body is able to reproduce the whole. It is true that the power of 

 regeneration is always limited to a greater or less extent according 

 to the species. But there is no evidence whatever that such limita- 

 tion arises through specification of the nuclei by qualitative division, 

 and, as will appear beyond, its explanation is probably to be sought 

 in a very different direction. 



F. On the Nature and Causes of Differentiat 



ION 



We have now cleared the ground for a restatement of the problem 

 of development and an examination of the views opposed to the \ 

 Roux-Weismann theory. After discarding the hypothesis of quali- j' 

 tative division the problem confronts us in the following form. If 

 chromatin be the idioplasm in which inheres the sum total of heredi- 

 tary forces, and if it be equally distributed at every cell-division, how 

 can its mode of action so vary in different cells as to cause diversity 

 of structure, i.e. dijferentiation ? It is perfectly certain that differen- 

 tiation is an actual progressive transformation of the egg-substance 

 involving both physical and chemical changes, occurring in a definite 

 order, and showing a definite distribution in the regions of the Q.^g. 

 These changes are sooner or later accompanied by the cleavage 

 of the Q.g^ into cells whose boundaries may sharply mark the 

 areas of differentiation. What gives these cells their specific char- 

 acter.'' Why, in the four-cell stage of an annelid Qg^, should the 

 four cells contribute equally to the formation of the alimentary canal 

 and the cephalic nervous svstem, while only one of them (the left- 

 hand posterior) gives rise to the nervous system of the trunk-region 

 and to the muscles, connective tissues, and the germ-cells.' (Figs. 

 171, 188, ^.) There cannot be a fixed relation between the various 

 regions of the &g^ which these blastomeres represent and the adult 

 parts arising from them ; for in some eggs these relations may be 

 artificially changed. A portion of the ^gg which under normal con- 

 ditions would give rise to only a fragment of the body will, if split off 

 from the rest, give rise to an entire body of diminished size. What 

 then determines the history of such a portion } What influence 

 moulds it now into an entire body, now into a part of a body } 



De Vries, in his remarkable essay on Intracellular Pangenesis 

 ('09), endeavoured to cut this Gordian knot by assuming that the 

 character of each cell is determined by pangens that migrate from 



