970 THE AMERICAN NATURALIST. [Vor. XXXV. 
new ones, and those that have thus been organized are sup- 
posed to attract others which are then deposited on the outer 
end and in turn become living structures. This idea is not 
fundamentally different from that held at present as to the 
growth of a crystal, except in so far as the molecules are sup- 
posed to be organized by those in the old part or by those that 
have previously been altered, etc. But on this view we have no 
explanation of the process of morphallaxis, for, in this case, the 
change takes place only in the old part and throughout the old 
part. If we attempt to extend Pflüger's idea so that it may 
include these phenomena, we must suppose that a sort of 
recrystallization takes place in the old tissue or isolated piece; 
but this is really a very different conception from the former, 
for the process is not reorganization by apposition, but by com- 
plete working over of the entire piece into a new whole. This 
idea is entirely foreign to our conception of crystal growth, and 
also to the conception entertained by Pflüger. I have also 
pointed out that even in some cases in which a new part is 
added to the old one there are grave difficulties in the way of 
accepting Pflüger's view, especially in the case of the planarian, 
in which the new head does not appear in the middle of an 
anterior oblique exposed edge, but to one side of the middle 
line, where the influence of the old part would produce, on 
Pflüger's view, a very different result. 
Finally, I wish to discuss a problem that is at present one 
of the most difficult questions in connection with the process 
of regeneration. I mean our idea of polarity or polarization of 
the living material of the animal and plant. ‘The term was 
first used by Allman in 1864 to mean that a new anterior end 
appears on that part of a piece that lay nearest to the old 
anterior end, and a posterior end appears on that part that 
lay nearest the old posterior end. This rule is followed in a 
great many cases, but it is not invariable. In certain forms an 
organ different in kind from the one removed has been found 
to develop. The first case of the kind was described by 
Bonnet, who found when certain fresh-water. worms were cut 
in two that the posterior piece produced at its anterior end, | 
not a new head, but another new tail. Loeb also has described 
