OF MONOTREMATA AND MARSUPIALIA. 
473 
villi. I have not yet tried to trace the deposition of the shell to any special glands. 
From the fact that these villi become longer as the egg increases in size it is clear 
that the shell does not thicken at the expense of the albumen layer inside. 
iii. In the uterus. —The egg, arrived in the uterus, has already received its full 
complement of membranes. The albuminous investment and the shell have added 
1*5 mm. to the egg’s diameter. The egg continues to increase in diameter after its 
arrival in the uterus, and by the time it has reached a diameter of 6'5 mm. the albumen 
layer has entirely disappeared, and the vitelline membrane thus comes to lie close to 
the shell. (Plate 30, figs. 3 and 4.) Meanwhile both shell membrane and vitelline 
membrane have again increased in thickness. (Plate 30, fig. 2, sh. and vrn.) Another 
layer, which stains darkly, present in the ovary after the pro-albumen appeared as the 
very delicate line containing granules, already described, lies inside the vitelline 
membrane, I look on it as a coagulum formed by reagents from the nutritive fluid 
entering the ovum. In the uterus it becomes very conspicuous on the disappearance 
of the albumen, and its presence enables the opening of the blastopore to be traced. 
The egg, on leaving its follicle, measured 2‘5 mm. to 3 mm. in diameter. When it 
is laid it measures 15 mm. by 12 mm. 1 
This enormous increase is due to the absorption of fluid from the walls of the 
uterus. The process shows itself by the continually increasing quantity of darkly 
staining coagulum inside the vitelline membrane and through the body of the yolk. 
Up to the close of segmentation this layer is thicker over the blastoderm than else¬ 
where. In many sections mounted in balsam the line between the vitelline mem¬ 
brane and this coagulum is not distinct, and the two appear as one thick vitelline 
membrane. 
The fluid layer betrays its nature, however, by passing in between the cells, and 
into an opening of the blastopore. 
The earliest stage of the shell membrane has been already described. Some of 
the stages it passes through before being laid are shown on Plate 30 (figs. 2, 3, and 
4, sh.). In Echidna I have not detected any calcic salts in the shell after laying, but 
on treating the shell of Ornithorhynchus with dilute hydrochloric acid a considerable 
quantity of gas is given off. When fresh-laid, the egg has a thickness of - 5 mm. and 
is of an opaque white colour; the cones figured on Plate 30 (fig. 4, sh.) are directly 
derived from the fine villi on the outside of the young shell membrane. 
2. — Marsupialia. 
i. In the ovary .—The development of the membranes just traced in Mono- 
tremata proceeds in exactly the same way in Pliascolarctos cinereus up to the stage 
when the yolk granules begin in the Monotremata to become the characteristic yellow 
* The laid eggs of both Echidna and Ornithorhynchus vary somewhat in size. I have a normal 
Echidna egg as small as 13 mm. by 12 mm. 
MDCCCLXXXVII.—B. 3 P 
