954 THE AMERICAN NATURALIST. [VoL. XXXV. 
at once as a whole, z.e, the isolated blastomere divides in the 
same way that the whole egg divides, and not as it would have 
divided had it remained in contact with its fellow. The blas- 
tula is closed from the beginning. Later, as in the other cases, 
a whole embryo develops. 
These and other experiments seem to show that the form of 
cleavage of the egg is the result of an arrangement or structure 
of the protoplasm, and that in some cases this same arrange- 
ment is retained after the separation of the blastomeres; while 
in other cases the protoplasm may rearrange itself at once 
into a new whole, in which the arrangement of the parts is 
symmetrical and like that of the whole egg. The blastomere 
divides in consequence in the same way as does the egg. 
The closing in of the half blastula of Echinus presents cer- 
tain obvious resemblances to the closing in of pieces of adult 
animals. Several writers have supposed that in both cases 
this change is due to a simple physical rearrangement of the 
material, but it is, I think, not altogether improbable that at 
least one other factor is also present. I base my opinion on the 
study that I have made of the closing in of pieces of Tubularia, 
of hydra, and of the closing of large wounds made in the side 
of very young tadpoles. While the closing in seems to be the 
result, in part, of the physical property of the semifluid sub- 
stance to become round, or more symmetrical, it seems also 
to involve certain contractile phenomena of a different sort. 
The cells that are in contact draw towards each other as à 
result of their own movement or contractility. There may be 
also certain tactic phenomena present. At least, if we cannot 
show positively that some such factors may be at work we have 
at least no right to exclude them in the present state of our 
knowledge as possible factors in the result. 
After the closing of the half blastula has taken place we find 
that the piece no longer behaves as a part, but as a new whole. 
We must assume that it has become a symmetrical or whole 
structure. Let us look somewhat further into this question. 
We have seen that in the case of the egg the change that 
takes place in a piece, making the piece symmetrical, may be 
brought about by a simple physical rearrangement of the 
