ated sphere like the _Volvox_ has no parts of its
periphery unlike other parts; and it is not to be expected that it
should have; seeing that as it revolves in all directions, it does not,
in traversing the water, permanently expose any part to special
conditions. But when we come to creatures that are either fixed, or
while moving, severally preserve a definite attitude, we no longer find
uniformity of surface. The gemmule of a Zoophyte, which during its
locomotive stage is distinguishable only into outer and inner tissues,
no sooner takes root than its upper end begins to assume a different
structure from its lower. The free-swimming embryo of an aquatic
annelid, being ovate and not ciliated all over, moves with one end
foremost; and its differentiations proceed in conformity with this
contrast of circumstances.

The principle thus displayed in the humbler forms of life, is traceable
during the development of the higher; though being here soon masked by
the assumption of the hereditary type, it cannot be traced far. Thus the
"mulberry-mass" into which a fertilized ovum of a vertebrate animal
first resolves itself, soon begins to exhibit a difference between the
outer and inner parts answering to the difference of circumstances. The
peripheral cells, after reaching a more complete development than the
central ones, coalesce into a membrane enclosing the rest; and then the
cells lying next to these outer ones become aggregated with them, and
increase the thickness of the germinal membrane, while the central cells
liquefy. Again, one part of the germinal membrane presently becomes
distinguishable as the germinal spot; and without asserting that the
cause of this is to be found in the unlike relations which the
respective parts of the germinal membrane bear to environing influences,
it is clear that we have in these unlike relations an element of
disturbance tending to destroy the original homogeneity of the germinal
membrane. Further, the germinal membrane by and by divides into two
layers, internal and external; the one in contact with the liquefied
interior part or yelk, the other exposed to the surrounding fluids: this
contrast of circumstances being in obvious correspondence with the
contrast of structures which follows it. Once more, the subsequent
appearance of the vascular layer between these mucous and serous layers,
as they have been named, admits of a like interpretation. And in this
and the various complicatio