472 
MR. W. H. CALDWELL ON THE EMBRYOLOGY 
of the follicular epithelium constitutes a well-marked stage, and will be described as 
the third period. 
The follicle has by this time come to project from the ovary so much that, in many 
cases, there is a marked pedicle of attachment. The follicular epithelium, beginning 
in the region underlying the projecting surface of the follicle, divides rapidly, and 
the cells, becoming enormously enlarged, secrete a dense homogeneous substance on 
their inner face, next the vitelline membrane (Plate 29, fig. 4, ch). This process of 
secretion goes on until the whole egg is suspended in a dense layer, which, from its fate, 
I shall speak of as the “ pro-albumen.” The follicle, being now clasped by the open 
mouth of the Fallopian tube, bursts, and the egg is received into the Fallopian tube. 
A few cells of the follicular epithelium remain attached to the pro-albumen. The 
majority of the follicular cells remain behind, inside the follicle, and there they 
multiply so rapidly that very soon the whole cavity of the follicle is occupied by 
gigantic cells derived from continued division of the follicular epithelium, between 
which connective tissue cells have also grown in from the walls of the follicle itself. 
The further changes that take place in this “ corpus luteum ” need not be described. 
ii. In the Fallopian tube. —I found one female Ornithorhynchus with two eggs in the 
dilated end of the Fallopian tube (infundibulum). Both eggs had begun to segment, 
and one had already acquired eight segmentation nuclei. Each egg was enclosed by 
its thin vitelline membrane (Plate 30, fig. 1 ; Plate 31, fig. 3, m), and surrounded by 
the pro-albumen. Here and there traces of the follicular cells, which remained 
attached to their own secretion, are visible in the sections. A few darkly staining 
granules were visible all round the ovum, connected with each other by a fine, 
darkly staining line, just inside the vitelline membrane. This layer was already 
visible in some sections through ripe ova, after the pro-albumen was formed. 
The unsegmented egg of Echidna (a medium section through the germinal disc 
of which is figured on Plate 31, fig. 1) was taken from the lower part of 
the Fallopian tube. In this egg the vitelline membrane has already increased in 
thickness to about '0016 mm., and the pro-albumen has also increased in thickness 
until it becomes the definite albumen layer by absorption of fluid in the Fallopian 
tube. In the living egg this albumen has the same appearance as the albumen of 
a Hen’s egg, and, treated with alcohol, it comes down as a granular precipitate. 
(Plate 30, fig. 3, alb. ; Plate 31, fig. 1 , cdb.) The albumen in hardened ova varies in 
thickness at different places by reason of its fluid nature. That the section figured in 
Plate 30, fig. 3, happens to have less thickness of albumen than the pro-albumen from 
which it arose is explained in this way. 
Outside the albumen there is now a new structure—the shell membrane (Plate 31, 
fig. 1 , sh .)—which first appears in the lower part of the Fallopian tube. The shell 
membrane is of a tough parchment-like consistency, and does not stain with haemato- 
xylin or borax carmine. On the outside it has a roughened surface, which in 
section (Plate 31, fig. 1 , sh.) is seen to be caused by the presence of numerous fine 
