439 



in which the physiological units are equilibrated, cannot be 

 represented within the small bulk of the embryo. In many 

 minute organisms, where the whole mass of physiological units 

 required for the structure is present, the very thing does take 

 place which it is above implied ought to take place. The mass 

 builds itself directly into the complete form. This is so with 

 Acari, and among the nematoid Entozoa. But among higher 

 animals such direct transformations cannot happen. The mass 

 of physiological units required to produce the size as well as 

 the structure that approximately equilibrates them, is not all 

 present, but has to be formed by successive additions addi- 

 tions which in viviparous animals are made by absorbing, 

 and transforming into these special molecules, the organizable 

 materials directly supplied by the parent, and which in ovi- 

 parous animals are made by doing the like with the organ- 

 izable materials in the " food-yelk,'' deposited by the parent in the 

 same envelope with the germ. Hence it results that, under such 

 conditions, the physiological units which first aggregate into the 

 rudiment of the future organism, do not form a structure like that 

 of the adult organism, which, when of such small dimensions, does 

 not equilibrate them. They distribute themselves so as partly to 

 satisfy the chief among their complex polarities. The vaguely-dif- 

 ferentiated mass thus produced cannot, however, be in equilibrium. 

 Each increment of physiological units formed and integrated by it, 

 changes the distribution of forces ; and this has a double effect. It 

 tends to modify the differentiations already made, bringing them a 

 step nearer to the equilibrating structure; and the physiological 

 units next integrated, being brought under the aggregate of polar 

 forces exercised by the whole mass, which now approaches a step 

 nearer to that ultimate distribution of polar forces which exists in 

 the adult organism, are coerced more directly into the typical struc- 

 ture. Thus there is necessitated a series of compromises. Each 

 successive form assumed is unstable and transitional : approach to 

 the typical structure going on hand in hand with approach to the 

 typical bulk. 



Possibly I have not succeeded by this explanation, any more than 

 by the orig'nal explanation, in making this process " representable 

 in thought." It is manifestly untrue, however, that I have, as 

 alleged, re-introduced under a disguise the conception of a " vital 

 principle." That I interpret embryonic development in terms of 

 Matter and Motion, cannot, I think, be questioned. Whether the 

 interpretation is adequate, must be a matter .of opinion ; but it is 

 clearly a matter of fact, that I have not fallen into the inconsistency 

 asserted by your reviewer. At the same time I willingly admit that, 

 in the absence of certain statements which I have now supplied, he 

 was not unwarranted in representing my conception in the way that 

 he has done. 



