474 
MR, W. H. CALDWELL ON THE EMBRYOLOGY 
yolk spheres (compare Plate 29, fig. 1, with Plate 29, fig. 5). In Phascolarctos, as in 
Monotremata, the delicate membrane surrounding the youngest ovum gradually 
changes into a distinct and strong membrane surrounding the ripe ovum. The ripe 
ovum of Phascolarctos, measuring T7 mm., resembles an ovum of Echidnai or 
Ornithorhynchus measuring '25 mm. The follicular epithelium throughout the ovarian 
period is connected with the ovum by numerous processes perforating the vitelline 
membrane, along which the yolk granules pass into the ovum. 
The youngest ova of Phascolarctos exactly resemble the youngest already described 
in Monotremata. The single layer of follicular cells soon becomes columnar, and also, 
by division, several rows deep. A cavity appears between the cells, in the same way 
as in Placentalia, and soon a typical Graafian follicle with its “ discus proligerus ” is 
formed. 
The follicle of Phascolarctos grows very much larger than in most Marsupialia. In 
Phascolarctos the ripe follicle is in many cases attached to the ovary by a very 
slender pedicle. The follicle is elliptical in shape, and measures 10 mm. by 7 mm. 
It is thus enormously larger than the ripe follicle of Monotremata. 
The split in the layers of the follicular epithelium, which forms the cavity of the 
Graafian follicle, extends completely round the ovum, so that the ovum, still sur¬ 
rounded by the follicular epithelium several cells deep, now lies in the centre of the 
follicle, with the so-called “ liquor folliculi ” on all sides of it. The “ liquor folliculi ” 
contains numerous branched cells, connecting the epithelial lining of the follicle with 
that surrounding the ovum (Plate 2.9, fig. 5,fe.). 
ii. In the Fallopian tube. —I have no observations on the membranes in the Fallopian 
tube of Marsupials. The few young ova I obtained were used for other purposes. 
I was not aware of the existence of the pro-albumen at the time I found them. 
iii. In the uterus. —The ovum of Phascolarctos passes rapidly into the uterus, and 
the ovum figured on Plate 29, fig. 5, had only reached the stage of the first furrow of 
segmentation. In this ovum the same membranes are present as in the corresponding 
stage of Echidna , with the striking difference that the follicle cells are still attached to 
a dense layer, similar to the pro-albumen in the Fallopian tube of Monotremata, 
This layer (Plate 29, fig. 5, ch.) lies immediately outside the vitelline membrane. 
Imbedded in it, and lying on the outside of it (Plate 30, fig. 5, fe,), are a large 
number of follicular cells, and, although their nuclei and nucleoli are still present, and 
suffice to prove their cellular nature, their general appearance, spherical form, and the 
absence of connecting processes between the cells, show that they are breaking down. 
Though I have failed to trace the formation of this layer in Marsupials from the 
follicular epithelium, its structure, position, and history all point to its being homo¬ 
logous with the pro-albumen of Monotremata, Towards the outside the cells and 
the pro-albumen are surrounded with fluid (Plate 30, fig. 5, alb.), and the outer part 
of the pro-albumen itself is less dense than the inner. 
Enclosing the whole egg is a thin transparent membrane '0015 mm. thick, similar 
