THE EXTERNAL CONDITIONS OF DEVELOPMENT 323 



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 tjie 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 theory, 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 egg 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. 



H. THE EXTERNAL CONDITIONS OF DEVELOPMENT 



We have thus far considered only the internal conditions of devel- 

 opment which are progressively created by the germ-cell itself. We 

 must now briefly glance at the external conditions afforded by the 

 environment of the embryo. That development is conditioned by 

 the external environment is obvious. But we have only recently 

 come to realize how intimate the relation is ; and it has been espe- 



