47G 
MR. W. H. CALDWELL ON THE EMBRYOLOGY 
appear as such at considerable distance from the surface of the ovum. In the ripe 
egg there is only a very thin layer of fine granules and white yolk, between the 
yellow yolk and the vitelline membrane. This distribution appears to be due to the 
relative rapidity the formation of yolk granules bears to their absorption of fluid at 
any one period. The active protoplasm of Phascolarctos, like that of Monotremata, 
is aggregated to one pole of the ovum. There is a lens-shaped germinal disc in Phas¬ 
colarctos as in Monotremata (Plate 29, fig. 5, and Plate 31, fig. 1). 
The yolk of Phascolarctos never gets beyond the stage of white yolk. The white 
yolk immediately below the germinal vesicle in Monotremata is the only part of the 
ovum where oil (?) globules occur (Plate 31, fig. 1). The whole yolk of Phascolarctos 
is similar to the region below the germinal disc of Monotremes. The details of the 
segmentation and the exact behaviour of the cells to form the gastrula will be traced 
in Part II. I shall here only describe the general characteristics of segmentation in 
Monotremata and Marsupialia, and point out how the segmentation of Placentalia has 
been derived from this. The first furroV marks out the germinal disc into a larger 
and a smaller area (Plate 30, fig. 5, n l and n 2 ). The first furrow has therefore made 
the ovum bilaterally symmetrical. The second furrow appears at right angles to the 
first, and divides the germinal disc into four regions, two larger and two smaller 
(Plate 31, fig. 2, n lt n, 2 ). All these regions are connected with each other and with 
the yolk by protoplasmic processes passing across the furrows. These first four nuclei 
give rise by division to all the nuclei of the future embryo. No nucleus of the yolk 
which would explain spontaneous formation of “ yolk nuclei ” is present. So far the 
description applies equally to Phascolarctos and to Monotremata, but the ovum of 
Phascolarctos is no larger than that of Placentalia. 
That the first two segmentation furrows should fail to divide the ovum, shows that 
although the ovum has nearly regained its original alecithal condition, it still retains the 
secondary arrangement of protoplasm, induced by the yolk of its more immediate 
ancestors. 
Comparison of the Egg Membranes of Monotremata and Marsupialia with those of 
Placentalia. 
Prom the detailed account of the development of the egg membranes in Mono¬ 
tremata and Marsupialia ( Phascolarctos ) it is clear that there are two primary egg 
membranes in each group, the vitelline membrane and the pro-albumen, while a 
secondary egg membrane, the shell membrane, is added in the Fallopian tube and 
uterus. 
There is a strong presumption that the egg membranes of Placentalia are homologous 
with those of Monotremata and Marsupialia. I shall attempt to show that the three 
membranes I have found in Monotremata and Marsupialia are present in Placentalia. 
There are two egg membranes generally recognised in Placentalia, the zona pellucida 
and the vitelline membrane. 
