50 
ON A PLATYPUS EMBRYO, 
give rise to the appearance seen in surface view. The mesoderm 
extends out as a continuous plate of uniform thickness beyond the 
edge of the medullary plate, whereas the medullary plate itself 
shows a very noticeable thickening as compared with that of the 
other sections figured. 
The marked retardation in the formation of the medullary folds 
and in the folding off of the embryo is one of the most character- 
istic features of the embryo at this stage. This may be due, as 
suggested by our friend Prof. J. T. "Wilson, to the mechanical 
effect of the rapid imbibition by the ovum of nutritive fluid 
secreted by the uterine glands. 
The mature ovarian ovum, according to Caldwell,* measures 
only 2*5 to 3 mm. in diameter. After the entrance of the ovum 
into the Fallopian tube the shell membrane and proalbumen are 
added externally to the vitelline membrane, and at the same time 
the ovum is increasing in size by the absorption of fluid. The 
youngest stages in our possession are eggs in which segmentation 
has advanced to some extent ; they measure 5 mm. in diameter, 
and possess a distinct and resistent shell membrane separated 
from the vitelline membrane by a thin layer of proalbumen. As 
development proceeds this layer of proalbumen is soon wholly 
absorbed, and in the eggs under consideration the blastodermic 
vesicle tightly distended with fluid fitted closely around the inner 
surface of the fully formed shell, the vitelline membrane being no 
longer recognisable. During the rapid imbibition of fluid by the 
blastodermic vesicle, and the consequent increase in size of the 
whole ovum, the wall of the vesicle including the embryonal area 
is closely pressed against the surrounding vitelline and shell 
membranes. It therefore seems reasonable to suppose that under 
such conditions, only those structural features of the embryo are 
produced which do not involve any upgrowths of the wall of the 
vesicle. Once the definite shell is fully formed around the egg 
and no possibility exists of its obtaining a further supply of 
maternal nutritive material, the normal development of bodily 
* Phil. Trans. 1887. 
