OF MONOTREMATA AND MARSUPIALTA. 
475 
to the shell membrane of Monotremata (Plate 30, fig. 5, sh.). The farther changes 
undergone by these membranes enclosing the ovum are as follows :—The ovum soon 
increases in size, and in an ovum measuring '3 mm. the shell has become markedly 
thicker, viz., - 01 mm. The vitelline membrane also increases slightly in thickness, 
but the great increase in size of the ovum is due to the swelling up of the follicular 
secretion by the absorption of fluid from the walls of the uterus (Plate 30, fig. 7, alb.). 
Two stages are shown in section of this swelling up (Plate 30, figs. 6 and 7). All 
traces of follicular cells have vanished by this time. The further changes that take 
place with the formation of the blastodermic vesicle and development of the yolk sac 
are similar to those that occur in Monotremata at a corresponding age. The albumen 
soon disappears, and the vitelline membrane comes to lie close to the shell. No 
sooner is the albumen layer formed than it begins to pass through the vitelline mem¬ 
brane to nourish the ovum, and after the albumen layer has disappeared another 
layer has become very conspicuous inside the vitelline membrane. This layer is a 
coagulum formed from nutritive fluid on its way to feed the developing embryo, and 
shows its fluid nature, in the same way as pointed out in Monotremata, by passing 
into the open portion of the blastopore, and between the cells of the blastoderm. 
The distinction between the vitelline membrane, now considerably increased in thick¬ 
ness, and this coagulum is difficult to trace all round the ovum. 
This fact has led Selenka to describe the vitelline membrane as disappearing at an 
early stage, and the albumen as being directly continuous in development with the 
coagulum. The membrane, composed of the shell and the vitelline membrane, persists 
up to the stage when the blastodermic vesicle becomes fixed to the walls of the uterus. 
It is possible that it may persist for a longer period over the non-vascular unattached 
area of the yolk sac and allantois (vide my paper, ‘ Quart. Journ. Microsc. Sci.,’ 1884). 
The blastodermic vesicle enclosed in the membrane just described, measuring 15 mm. 
in diameter, floats freely in the uterus, and is exactly comparable to the laid egg of 
Monotremes. I have not found any trace of villi on the surface of the shell in 
Marsupials. 
B.—The Development of the Ova up to the First Stages in Segmentation. 
Monotremata and Marsupialia. 
Some of the changes that take place in the ovum have been described in dealing 
with the follicular epithelium. The ovum, while maintaining its character of a single 
cell, has become loaded with food material, which has been deposited in a horse-shoe¬ 
shaped mass round the germinal vesicle. This horse-shoe-like arrangement of the 
yolk is caused by its double mode of origin, and is common to all Ter te brat a with 
yolk-forming follicular epithelium. The different positions in which the yolk has 
been described as arising in meroblastic eggs may not be so different as has been 
supposed. At the end of the first period in Monotremata the yellow yolk spheres 
3 P 2 
