396 
thorhynchus, is of a spherical form, 
and very nearly fills the ovisac. The 
diameter of the germinal vesicle is to 
that of the ovum as 1 to 38. The 
vitelline fluid is rich in the number 
of its nucleated cells or granules, 
and the intermixed, clear, colourless 
oil-globules. The vitelline membrane 
is moderately thick, smooth, and 
highly refracting. The ovum is se- 
ted from the ovarian vesicle, or 
ining membrane of the ovisac, by a 
very small quantity of fluid and a 
Stratum of granules or cells. The 
proper tunic of the ovisac consists of 
a dense and very vascular layer of 
the ‘ stroma’ or proper tissue of the 
ovary, which is rather thicker and 
more distinctly laminated than in 
most Mammalia, and in this re- 
spect widely differs from the lax 
stroma of the ovary of the bird. The 
most important difference to be noted in the 
present comparison with the bird is the small 
size of the ovarian ovum, depending on the 
relatively scanty amount of vitelline matter, 
superadded in the ovary to the essential part of 
the ovum, the vesicula germinativa. 
It may be objected that the impregnated 
ovarian ova in birds rapidly augment in bulk 
as the time of dehiscence approaches, and that, 
although the ovaria of the Ornithorhynchus 
may have been investigated within a few days 
of the reception of the impregnated ovum into 
the oviduct, the changes occurring in such a 
period might much more nearly approximate 
the ovarian ovum of the Ornithorhynchus to 
the size and other conditions of that of the bird 
than in the instances above described. 
The following observations on the impreg- 
nated ovum in the uterus itself prove, how- 
ever, that no such approximation to the bird 
in regard to the proportion of yolk added to 
the ovarian ovum, or as respects the size of 
the ovum prior to dehiscence, is made by the 
Ornithorhynchus. 
For the acquisition of this important evi- 
dence in the question of the generation of the 
Monotremata science is indebted to the ex- 
ertions of Mr.Geo. Bennett, F.L.S., a Member 
of the Royal College of Surgeons in London, 
and Colonial Zoologist at Sydney, New South 
Wales. Three uteri, containing undeveloped 
ova, were transmitted by that gentleman to the 
Museum of the Royal College of Surgeons, 
in 1834, and with the sanction of the Board of 
Curators, were described by me in the Philo- 
sophical Transactions of that year.* 
In these specimens the left ovary only had 
taken on the sexual actions, but did not ex- 
ceed in size the same parts in the unimpreg- 
nated specimens above described. The right 
ovary had, however, become enlarged ; it mea- 
sured half an inch in length, a third of an 
inch in breadth, and was about half a line in 
* See vol. cxxiv. p. 555, and Physiol. Catalogue 
of the Hunterian Museum, vol. v. p. 112, No. 
3460 A. 
MONOTREMATA. 
Left uterus i i 
(Owen, Phil. Trans. 1834. ) 
ted, Ornithorhynchus. 
thickness : a few ovisacs, about the size of a 
small pin’s head, projected from the surface. 
The left ovary in each of the specimens was 
concealed by the thin membrane forming the 
expanded orifice of the oviduct, to which it 
was agglutinated by a coagulated secretion. 
In two of the specimens the left ovary pre- 
sented two empty ovisacs, or corpora lutea 
(fig. 192, b 6), corresponding with the num- 
ber of ova found in the uterus. In the third 
specimen the left ovary presented two ovisacs 
still uncicatrized ; but only one ovum was con- 
tained in the uterus. In a fourth specimen 
three similar ovisacs were present, but the ova 
had been removed from the uterine cavity. 
The discharged ovisacs were of an elongated 
flask-shaped form about three lines in length, 
and two in diameter, with the margins of the 
orifice, through which the ovum and granular 
substance had passed, everted, with a slight 
contraction, resembling the neck of a flask, 
below the a pe On compressing these. 
ovisacs, small portions of coagulated sub- 
stance escaped. When longitudinally divided, 
they were found to consist of the same parts 
as the ovisac before impregnation, with the 
exception of the granular contents and gra- 
nular stratum; but the theca, or innermost 
parietes of the sac, was much thickened, and 
encroached irregularly upon the empty space, 
so as to leave only a cylindrical passage to the 
external opening. 
The impregnated Ornithorhynchus, in the 
uterus of which the two smallest sized ova 
were found, was shot on the evening of the 
6th of October, 1832, in the Yas river, Murray 
County, New South Wales. These ova were 
of a semitransparent white colour when 
but had lost that ap ce when exami 
at the Museum, to which they had been trans- 
mitted, in situ, with the uterus and surround- 
ing parts well preserved in spirits. The ova 
were situated at the upper part of the left 
uterus, and at the distance of about a line 
from each other. Each ovum was spherical in — 
form, and measured two lines and a half in 
diameter; they were of a deep yellow colour, 
