EMBRYONIC DEVELOPMENT 145

with an eccentric cavity; this is the blastula stage of development,
and the cavity is the blastocoele.

The cells of the blastula are still all alike, except for inessential
differences such as the amount of yolk or pigment they contain,
which depends on the part of the original egg included in the cell.
Experiment shows, moreover, that their developmental capacities
are not yet differentiated. Although we can point to certain areas
of the blastula, and say that unless the normal course of develop-
ment is interfered with, this patch of cells will develop into- brain,
that into gills, and so on, their developmental fate has actually
still to be determined and can be altered experimentally. There is
one important exception to this statement, however—the region
of the grey crescent at the apex of the dorso-ventral gradient
field. The differentiation of the parts of the embryo starts from
this region and no other. For this reason, this portion of the
embryo has been called the 'organizer/

The grey crescent is at present a part of the wall of the hollow
blastula, made visible in many, but not all, Amphibia by its
colour. The next phase of development is the phase of gastrula-
tion, in which the region of the grey crescent passes from its
superficial position into the interior of the blastula. This is
accomplished by a complicated movement. A streaming motion
sets in, by which the cells of this region converge towards a
narrow area. Instead of piling up at this spot, however, the tissue
rolls inwards and works its way into the interior of the hollow
blastula and then spreads out again to form a sheet of tissue
underlying the upper roof of the blastocoele. This sheet is the
chorda-rnesodcrm, so called because the central strip of it will
become the notochord (the precursor of the backbone) and its
lateral parts will become mesoderm from which many of the internal
organs will develop.

The chorda-mesoderm is, then, the organizer, the material of
the grey crescent, which has passed (been invaginated) into the
interior of the hollow embryo in the phase of gastrulation. The
embryo is now in the gastrula stage.

The chorda-mesoderm is now stretched out as a strip of tissue
underlying the upper roof (ectoderm) of the hollow embryo. The
ectoderm cells immediately above the strip of chorda-mesoderm
then begin to differentiate themselves from the surrounding
ectoderm (which will become epidermis), and by a process which
need not be described, form themselves into the medullary plate,