Page 537 - My FlipBook

GENERAL ACCOUNT OF EMBRYONIC DEVELOPMENT. 547 ;

Outside these two layers the zona pellucida forms another coating.
It, however, does not play any essential part in the further develop-
FiG. 291.

Diagrammatic views of tlie Blast ode iiiiic Vesicle of a Rabbit on the Seventh Day (from Balfour, after
Kd. V. Beneden) : lu the left-hand figure the vesicle is seen from above; in the right-hand figure,
from the siile. The white iiatch (a//) is the germinal area, and the slight constriction {ge) marks
the limit to which the hypoblast has extended.
ment of the embryo, at least in so far as we are to con.sider the subject
and henceforth we will not mention it.
This embryonic, area is the result of the thickening of the hypoblast
at the point where the development of the primitive streak will pres-
FiG. 292.

Embryonic area of a Rabbit's Ovum on the Seventh Day (from Kolliker): The shaded part (ag) is the
embryonic area; oo is the region of the blastodermic vesicle immediately surrounding the embry-
onic area, into which the mesoblast has already spread, and in which blood-vessels will shortly
appear; jir, primitive streak; rf, medullary groove.