956 THE AMERICAN NATURALIST. |. [Vor. XXXV. 
prove, and we are led to ask, What is the nature of this sym- 
metry! or organization that exists in the protoplasm of the 
egg, and which may appear in the protoplasm of a part of the 
egg? The problem is all the more difficult to understand 
when we find that an egg that has divided on a radially sym- 
metrical plan may produce a bilateral embryo. It is probable, 
however, in such cases that a bilateral structure is really pres- 
ent from the beginning of the cleavage, but that the factors 
that are at work during the cleavage are not necessarily those 
that determine the bilateral organization of the embryo. In 
some cases, as in the frog's egg, a careful examination even of 
the early cleavage stages shows that a bilateral organization is 
present in the protoplasm, although the form of the cleavage 
may appear in many cases to be radially symmetrical. 
In order to answer our question in regard to the nature of 
the organization we should have to know more of the nature of 
development itself. All we can do at present is to examine 
some of the implications that are involved in the assump- 
tion, and at least attempt to make clear our position. The 
new axial relations that are established in the piece present 
certain interesting relations. If the original organization was 
a bilateral one that corresponded, let us assume, more or less 
with the plane of the first division, then an isolated blastomere 
has at first only the organization of a half, or rather contains 
the factors that lead to a half development. In other words, 
it has no median plane of symmetry, yet later such a median 
plane is established. It is this change, taking place in the 
isolated part, that we are forced to assume, that gives us one of 
the most interesting and also important problems with which the 
student of experimental embryology has to deal. We know of 
nothing similar taking place in inorganic nature. The most obvi- 
ous change that may seem to approach this is in the formation 
of an entire crystal from a piece, but in this case there is never 
any change in the position of the original axes that the piece 
has inherited from the old crystal; there is no rearrangement 
1 The term “symmetry " does not, perhaps, express the idea entirely, since an 
egg with an asymmetrical cleavage must be regarded as having also a definite 
organization. 
