THE NUCLEUS IN LATER DEVELOPMENT 



427 



assumes a migration of pangens from nucleus to cytoplasm, the 

 character of the cell being determined by the nature of the mi^a'at- 

 ing pangens, and these being, as it were, selected by circumstances 

 (position of the cell, etc.). But, as already pointed out, the pangen- 

 hypothesis should be held quite distinct from the purely physiologi- 

 cal aspect of the question, and may be temporarily set aside ; for 

 specific nuclear substances may pass from the nucleus into the 

 cytoplasm in an unorganized form. Sachs, followed by Loeb, has 

 advanced the hypothesis that the development of particular organs 

 is determined by specific " formative substances " which incite cor- 

 responding forms of metabolic activity, growth, and differentiation. 

 It is but a step from this to the very interesting suggestion of 

 Driesch that the nucleus is a storehouse of ferments which pass 

 out into the cytoplasm and there set up specific activities. Under 

 the influence of these ferments the cytoplasmic organization is deter- 

 mined at every step of the development, and new conditions are 

 established for the ensuing change. This view is put forward only 

 tentatively as a "fiction" or working hypothesis; but it is certainly 

 full of suggestion. Could we establish the fact that the number of 

 ferments or formative substances in the nucleus diminishes with the 

 progress of differentiation, we should have a comparatively simple 

 and intelligible explanation of the specification of nuclei and the 

 limitation of development. The power of regeneration might then 

 be conceived, somewhat as in the Roux-Weismann theorv, as due to 

 a retention of idioplasm or germ-plasm — i.e. chromatin — in a less 

 highly modified condition, and the differences between the various 

 tissues in this regard, or between related organisms, would find a 

 natural explanation. 



Development may thus be conceived as a progressive transforma- 

 tion of the egg-substance primarily incited by the nucleus, first mani- 

 festing itself by specific changes in the cytoplasm, but sooner or later 

 involving in some measure the nuclear substance itself. This process, 

 which one is tempted to compare to a complicated and progressive 

 form of crystallization, begins with the youngest ovarian Q,^g and pro- 

 ceeds continuously until the cycle of individual life has run its course. 

 Cell-division is an accompaniment but not a direct cause of differen- 

 tiation. The cell is no more than a particular area of the germinal 

 substance comprising a certain quantity of cytoplasm and a mass of 

 idioplasm in its nucleus. Its character is primarily a manifestation 

 of the general formative energy acting at a particular point under 

 given conditions. When once such a circumscribed area has been 

 established, it may, however, emancipate itself in a greater or less 

 degree from the remainder of the mass, and acquire a specific char- 

 acter so fixed as to be incapable of further change save within the 

 limits imposed by its acquired character. 



