BY J AS. P. HILL AND C. J. MARTIN. 
51 
form by folding of the blastoderm is enabled to occur by the 
embryo making room for itself, so to speak, by the using up of 
the fluid contents of the vesicle. 
Against the view here put forward, it may be urged that in 
certain other forms there is a similar rapid increase in size of the 
blastodermic vesicle by the absorption of fluid, and yet there is 
no retardation in development. In Didelphys, for example, the 
blastodermic vesicle, according to Selenka, increases in one day 
from a diameter of 6 mm. to one of 15 mm., and at the end of this 
time the embryo is folded off, the medullary groove is closed and 
the amniotic folds developed. All these processes can, however, 
easily occur on an expanding blastodermic vesicle lying naked in 
the cavity of the uterus and devoid of any such mechanical 
obstacle as would be presented by the presence of a resistent shell 
membrane. 
Ectoderm. 
The ectoderm forms a continuous covering for the whole of the 
blastodermic vesicle. It consists, except in the regions to be 
subsequently mentioned, of a single layer of polygonal cells. Over 
the greater portion of the embryonic area the cells are much 
flattened, while in the head region of the embryo and in the 
extra-embryonic region of the wall of the vesicle they appear 
cubical in section. 
M e dull ar y plat e. — The medullary plate is, as already 
mentioned, still practically flat. Medullary folds are only present 
in the anterior region of the future fore-brain ; their appearance 
in this region is probably to be associated with the very early 
appearance of the optic grooves. The plate consists of elongated 
cells, the nuclei of which are situated at different levels simulating 
the appearance of several layers of cells. The lateral portions of 
the plate are thickest, and are connected by a median much 
thinner portion which sends down a keel-shaped process in 
some parts to meet the notochord. Along the median portion of 
the plate there runs a distinct groove — the " Riickenfurche." 
Beginning as a shallow groove slightly behind the anterior end of 
