OVIPARA. 199 
in addition, in the egg, a highly vascular membrane, which appears to be 
subservient to respiration; it is connected with the bladder, and is analo- 
and comes presently to cover the inner surface of the membrane of the shell. It is 
the chorion, and has numerous vessels ramifying on it, like the chorion of the sow, 
and connected in like manner with the fcetus. The blood of the umbilical artery is 
dark coloured, that of the vein bright. As incubation advances, the amnion enlarges, 
and comes in contact every where with the chorion. The albumen is all consumed, 
being taken into the vitellus, which is in a great measure absorbed; and what re- 
mains is taken, together with the sac, into the abdomen of the chick, and the parie- 
tes close over it. On the 21st day, the chick breaks the shell and escapes. By in- 
creasing or diminishing the temperature within a certain extent, the process may be 
somewhat accelerated or retarded. The eggs of large birds require a longer time to 
be hatched; those of the ostrich, for example, take six weeks. 
‘Hence it appears, that the vitellus and albumen contribute to the increment of 
the foetus, whilst the exterior membranes act as lungs, the air being transmitted 
through the pores of the shell. 
‘Sir Everard Home has lately investigated more minutely, by means of the 
microscope, the structure of the egg. The yolk is enclosed in two layers of mem- 
brane, the outermost very thin and delicate; the internal one, on the contrary, thick 
and spongy, with a small aperture, or deficiency, at one spot. Here the gelatinous 
molecule, from which the embryo is to be formed, is found placed on the surface of 
the yolk. Itis not quite so large as the opening in the membrane, and, therefore, 
seems to be surrounded by a margin, or an areola. In its passage along the oviduct, 
the yolk comes to be surrounded by the albumen, which presently is covered with a 
very fine membrane. During this descent are also formed the chalazes, which ter- 
minate in that double membrane which is added when the egg has reached the lower 
end of the oviduct, and which is distinct from, and exterior to the fine membrane 
already noticed. In four hours after the commencement of incubation, the outer 
edge of the areola had become enlarged, and that part next the molecule darker. 
One end of the molecule appeared like a white line, which is the rudiment of the 
embryo. In eight hours the white line had extended, and rudiments of the brain 
and spinal marrow could be discerned, surrounded by a membrane which afterwards 
becomes the amnion. The areola had extended, and the surface beyond the line 
which had formed its boundary had acquired the consistence of a membrane, and 
was circumscribed by a distinct line. This he calls the outer areola. In thirty-six 
hours, a vesicle had begun to protrude under the inner areola, apparently at the ter- 
mination of the spinal marrow. In sixty hours, auricles and ventricles were seen, 
the former filled with red blood; a trunk from the left ventricle gives off two large 
vessels which send branches over the whole areolar membrane. The vesicle some- 
what enlarged. In three days, the vesicle, still more enlarged, had forced its way 
through the external covering of the yolk, whereby a part of the albumen was ad- 
mitted to mix with the yolk. In four days, the vesicle was still larger, and its vessels 
contained red blood. On the fifth, it was of great size, very vascular, and its cavity 
contained a fluid. On the sixth, it had expanded, like a double nighteap, over the 
yolk, and its coverings were beginning to enclose the embryo. ‘This vesicle, Sir 
Everard, in another place, compares to the bag which, in the human subject, and in 
quadrupeds, is to become the urinary bladder. This increases with such rapidity 
that it bursts the amnion, and the arteries lying onits two sides are carried directly to 
the chorion, and there the placenta is formed in the empty space between the two 
edges of the chorion. Phil. Trans. 1823, Part II. p. 339. Vide also a paper by 
Dutrochet on the envelopes of the foetus, in the Bull. de la Soc. de Med. 1819, No. 
8. And Mem. de la Societé d’Emulation. Tom. VIII. pp. 3,760. 
‘ The eggs of fishes have a general resemblance to those of fowls, and consist of 
a vitellus and albumen, with their membranes; but in place of being furnished with 
a shell they have a tough, or sometimes a horny covering; and some, as those of the 
shark, torpedo, &c. are quadrangular in shape. The yolk is connected to the intes- 
tines of the foetus, and its membrane is very vascular. As in fowls, so in fishes, it 
is ultimately enclosed within the abdomen of the young. In the skate, numerous 
blood vessels are formed in the albumen, which supply the place of gills, and are 
supposed by Dr. Monro, to be afterwards covered and converted into gills. The two 
functions of a placenta, then, are still more distinctly fulfilled here than even in 
