966 THE AMERICAN NATURALIST. [Vor. XXXV. 
II. 
A year ago! I reviewed the different hypotheses that have 
been advanced to account for the phenomena of regeneration 
and I pointed out that if the development of a part of an egg, 
or of an embryo, is also a process of regeneration, as I believe 
to be the case, the same hypotheses ought to apply also to the 
development of these parts. It may be, therefore, worth while 
to see how far these hypotheses may account for the develop- 
ment of pieces of the egg or embryo. 
Bonnet extended his theory of praformed germs, primarily 
invented to explain the development of the embryo from the 
egg, to include the phenomena of regeneration. At that time 
the development of a part of an egg was not known, but there 
is no obstacle at present to applying the same interpretation to 
the development or regeneration of parts of the egg and embryo. 
In fact, Weismann, who also believes in a theory of praeforma- 
tion both for egg-development and for development of pieces 
of adult organisms, has applied the same view to the develop- 
ment of parts of an egg. The theory of praformed germs as 
held during the seventeenth and eighteenth centuries need no 
longer be seriously discussed in the form in which it was then 
expressed, since we have abundant evidence to disprove the 
view; but the same conception has appeared again in a more 
insidious form in our own time, and has been applied not only 
to development of the egg, but also to the regeneration of both 
the adult and embryo. I shall not attempt to repeat here the 
arguments in favor of or against this view, but shall confine 
what I have to say to those points that bear on our present 
examination. The modern form of the doctrine of prefor- 
mation as held by Weismann and his school is as follows. 
There exists in the nucleus of the egg praformed germs 
that correspond part for part to the later structures that 
develop from the egg. The process of development is a 
process of sorting out of the nuclear germs to different parts 
of the embryo, and later, by the action of the germs con- 
tained in each nucleus on the protoplasm of each cell, they 
1 Woods Hole Lectures, 1899. 
