470 
MR. W. H. CALDWELL ON THE EMBRYOLOGY 
bare Membran zu biklen.” This statement is of interest in connection with Selenka’s 
(57) similar account of the formation of an egg membrane in the Opossum. The ovary 
of Marsupialia has been described and figured in several genera. Sir B. Owen 
observed the enormous stalked follicles of Phascolomys. Poulton’s account of the 
development of the Graafian follicle in Phalangista is the only paper on the subject known 
to me. He noticed that the cumulus proligerus separated from the wall of the follicle. 
Selenka (57) found follicular cells in the Fallopian tube of the Opossum. Observa¬ 
tions on the uterine egg membranes of Monotremata have been made by Sir B. Owen, 
who in 1865 distinguished two membranes, an outer “ chorion ” and an inner “ mem- 
brana vitelli,” but he wrongly stated that the “ increase of the size of the uterine 
over the ripe ovarian ovum was due to an increase of fluid between the chorion and 
the membrana vitelli.” Selenka is the only observer of the early uterine stages of 
the egg membranes of Marsupialia. He derives the outer membrane of the uterine 
ovum directly from the follicular epithelium, and further states that the zona pellu- 
cida soon disappears, which results in his confusing the albumen layer of the early 
stages with the coagulum surrounding the embryo in later stages. Selenka, in 
his preliminary note on the Opossum (56), says, “ Die Eier halten die Mitte zwischen 
den meroblastischen unci holoblastischen. ” The exact meaning of this statement is made 
clear by the beautiful drawings in his Memoir. 
A.— The Egg Membranes. 
1.— Monotremata. 
i. In the ovary .—Bound the very young ova a fine membrane is present between 
the single row of follicular cells and the ovum. I shall speak of this as the vitelline 
membrane, but whether it is produced by the ovum or the follicular epithelial cells I 
have not attempted to determine. 
With the growth of the ovum the vitelline membrane increases enormously in 
thickness. In ova of ‘32 mm. diameter it reaches its maximum thickness of 
'016 mm. With the formation of the yellow yolk spheres the vitelline membrane 
again becomes thinner, till in the ripe ovarian ovum it has no longer a measurable 
thickness. The vitelline membrane is perforated by protoplasmic processes connecting 
the protoplasm of the ovum with that of the follicular epithelium. 
Bound very young ova the follicular epithelium consists of a single layer of flattened 
cells, each of which, like the ovum itself, consists of clear protoplasm nearly free from 
granules. 
The difference between a ripe ovum and such an unstainable cell is caused by the 
addition of food material, which is formed in the cells of the follicular epithelium as 
well as in the ovum itself, and appears first as minute granules in the neighbourhood 
of the nuclei and germinal vesicle. Every stage is to be found, from the most minute 
granules up to the largest yellow yolk spheres of the ripe ovum. 
