960 THE AMERICAN NATURALIST. | [Vor. XXXV. 
in the egg; perhaps to a lack of lability in the protoplasm, 
so that it fails to reorganize itself into a new whole. The 
result is not due to the smallness of the piece, for under 
certain conditions, not yet understood, a smaller piece than 
a half or a fourth of the egg may make a whole structure. 
We must, therefore, look upon the egg of the ctenophore as toti- 
potent in all its parts, in the same way that the sea-urchin's 
egg is totipotent, but an incomplete embryo develops from a 
piece, since the egg after fertilization has not the power, under 
ordinary circumstances, of rearrangement or regulation. It is 
conceivable that at a different temperature or in a slightly dif- 
ferent medium the protoplasm of the piece might rearrange 
itself so that an entire embryo would develop. 
If we think of the embryo as organized in the protoplasm of 
the egg in the sense of there being present a definite structural 
arrangement of the protoplasm, then when the blastomere of 
the ctenophore egg is isolated there is simply a lack of change. 
If, on the other hand, we think of the egg at the beginning of 
cleavage as being organized only in so far as the cleavage is 
concerned, the later organizations appearing afterwards, then 
we must think of the half organization of the embryo as being 
induced by the half form of the cleavage, or the half form of 
the isolated part. The half development would, in this case, not 
be due to a lack of change, but to the appearance after cleavage 
of a half organization. If we adopt the latter alternative we 
can compare the development of the one-half ctenophore embryo 
with the production of a part of a hydranth in a small piece of 
Tubularia. In both cases a part of a structure is formed on 
account of the form or size of the piece in material that is toti- 
potent. If, on the other hand, we adopt the first alternative we 
find the two processes different, unless it could be shown that 
towards the distal end of the piece of Tubularia a new hydranth 
may be organizing before the old hydranth is thrown off. The 
removal of a piece taking place at this time, each piece con- 
tinues to develop in the determined direction. Since, however, 
only distal organs develop from such pieces at several levels 
we see that this view is insufficient by itself to give à satis- 
factory solution. 
