OF MONOTREMATA AND MARSUPIALIA. 
479 
not be homologous with the albumen or the shell of Elasmobranchii. It seems more 
probable that the difference between the shell of Elasmobranchii and that of Myxine 
is due to the fact that in Elasmobranchii the pro-albumen absorbs fluid in the course 
of its passage down the oviduct. 
The shell membrane presents another difficulty. I have shown that in Marsupialia 
it does not increase at the expense of the albumen. 
Cunningham states that the shell of Myxine arises directly from the pro-albumen, 
and others have imagined a similar origin for it in Elasmobranchs and Sauropsida. 
Yon Baer compared the albumen of a Frog’s egg to the shell of a Bird’s. 
These considerations show that further investigations are necessary in both Ichthyo- 
psida and Sauropsida before the three membranes can be traced through Yertehrates. 
Summary. 
In Monotremata and Marsupialia— 
I. There is a vitelline membrane which, appearing between the young ovum 
and the follicular epithelium, persists until hatching in Monotremata, and 
until late uterine stages in Marsupialia. 
II. There is a second primary egg membrane secreted by the follicular epithe¬ 
lium shortly before the ovum leaves the ovarian follicle—the pro-albumen. 
III. The pro-albumen, by absorption of fluid in the Fallopian tube and uterus, 
becomes the albumen layer outside the vitelline membrane. 
IY. A secondary egg membrane—the shell membrane—is found in the Fallopian 
tube, and becomes thicker in the uterus. 
Y. The albumen soon disappears, and the vitelline membrane comes to lie next 
the shell. 
YI. The ovum absorbs fluid from the uterus, and increases in Monotremes from 
about 3 mm. to 15X13 mm. 
VII. A layer, simulating the appearance of a membrane, inside the vitelline mem¬ 
brane, is a coagulum formed by reagents from the nutritive fluid passing 
into the ovum. 
VIII. There is a germinal disc, and the ovum undergoes a partial segmentation in 
Monotremes and Phascolarctos. 
In Placentalia— 
IX. The vitelline membrane has generally been known as part of the zona pellu- 
cida. 
X. The pro-albumen is probably represented by the outer “ granular layer ” of 
the zona pellucicla. 
XI. The shell membrane has not been recognised in its early stages, except 
by Barry. The “ villous chorion ” (non-cellular, Bisciioff) is probably 
partly derived from a true shell membrane. 
