ZOOLOGY.- 847 
hang into the yolk. There can be no doubt that they have 
the office of absorbing the yolk, and conveying it into the 
veins of the yolk-bag; where it is assimilated to the blood, 
and applied to the nutrition of the chick. 
The first parts which can be discerned in the uterus after 
impregnation, are the membranes (involucra) of the ovum; 
in which (the marsupial animals excepted) the embryo itself 
becomes visible after a certain period. By means of the 
navel-string the foetus is connected to these membranes, and 
consequently to the uterus of the mother; from which its 
nourishment is derived until the time of birth. 
The mode of connexion of the pregnant uterus with the 
membranes of the ovum, and thereby with the embryo itself, 
displays three chief differences in the various mammalia. 
Either the whole external surface of the ovum adheres to 
the cavity of the uterus; or the connexion is effected by 
means of a simple placenta; or by numerous small placentae 
(cotyledons). 
The first kind of structure is observed in the sow; and is 
still more manifest in the mare. 
In those animals of this class, where the embryo is nou¬ 
rished by means of a placenta, remarkable varieties occur in 
the several species; sometimes in the form and successive 
changes of the part, sometimes in the structure of the organ, 
as being more simple or complicated. Thus in most of the 
digitated mammalia, as well as in the quadrumana, the pla¬ 
centa has a roundish form; yet it consists sometimes of two 
halves lying near together; and in the dog, cat, martin, &c. 
it resembles a belt (cingulum or zona). Its form in the pole¬ 
cat is intermediate between these two structures; as there 
are two round masses joined by an intervening narrower 
portion. 
In several species of digitated mammalia the external sur¬ 
face of the placenta is provided with a white and apparently 
glandular body (corpus glandulosum Everardi, or subpla¬ 
centa,) smaller than the proper placenta by which it is in¬ 
closed. In proportion as the embryo becomes more mature, 
this part admits of more easy separation from the placenta. 
The placenta of the bisulca is divided into numerous coty¬ 
ledons ; the structure of which is very interesting, as it eluci¬ 
dates the whole physiology of this organ. The parts desig¬ 
nated by this appellation are certain fleshy excrescences, 
(glandulae uterinae) produced from the surface of the impreg¬ 
nated uterus, and having a corresponding number of floccu- 
lent fasciculi of blood-vessels, (carunculae) which grow from 
the external surface of the chorion implanted in them. Thus 
the uterine and foetal portions of the placenta are manifestly 
distinct from each other, and are easily separable as the foetus 
advances to maturity. The latter only are discharged with 
the after-birth, while the former, or the cotyledons gradually 
disappear from the surface of the uterus after it has parted 
with its contents. The number and form of these excre¬ 
scences vary in the different genera and species. In the 
sheep and cow they sometimes amount to a hundred. In the 
former animal and the goat, they are, as the name implies, 
concave eminences; while on the contrary, in the cow, deer, 
&c. their surface is rounded or convex. 
In the foal, as in the child, the chord possesses a single 
umbilical vein ; whilst most other quadrupeds have two, 
which unite however into a common trunk near the body of 
the foetus, or just within it. 
The amnion, or innermost of the two membranes of the 
ovum, which belongs to the pregnant woman, as well as to 
the mammalia, is distinguished in some of the latter, as for 
instance in the cow and mare, by its numerous blood-vessels; 
while on the contrary, in the human subject, it possesses no 
discernible vascular ramification. 
Between the chorion and amnion there is a part found in 
most pregnant quadrupeds, and even in the cetacea, which 
does not belong to the human ovum, viz. the allantois, or 
urinary membrane. The latter name is derived from the 
connexion which this part has, by means of the urachus, 
with the urinary bladder of the foetus; whence the watery 
fluid, which it contains, has been regarded as the urine of the 
animal. The term allantois has arisen from the sausage-like 
form which the part possesses in the bisulca and the pig; 
although this shape is not found in several other genera and 
species. Thus, in the hare, rabbit, guinea-pig, &c. it re¬ 
sembles a small flask; and it is oval in the polecat. It 
covers the whole internal surface of the chorion in the soli- 
dungula, and therefore incloses the foal with its amnion. It 
contains most frequently in these animals, (although not 
rarely in the cow) larger or smaller masses of an apparently 
coagulated sediment in various forms and number, which 
has been long known by the singular name of the horse- 
venom or hippomanes. 
Some orders and genera of mammalia resemble the human 
subject in having no allantois; as the quadrumana and the 
hedgehog: nay, in the latter animal, the urinary bladder has 
no trace whatever of urachus; which even exists in a certain 
degree in the human subject; but its fundus is perfectly 
spherical in the foetus. 
There is in the hedgehog, as well as in the dog, cat, and 
others, a peculiar part called the tunica erythroides, situated 
between the chorion and amnion like the allantois, for which 
it might easily be mistaken on the first view. It contains a 
watery fluid at the commencement of pregnancy, but is 
easily distinguished from an allantois, as it is not joined to 
the fundus of the bladder by the urachus, but is connected 
by means of the omphalo-mesenteric veins, with the mesen¬ 
teric blood-vessels of the foetus. This connexion constitutes 
a resemblance on one hand to the yolk-bag of the incubated 
bird, and on the other hand to that remarkable vesicula um- 
bilicalis, which is observable in the early months of preg¬ 
nancy. The tunica erythroides, as well as that vesicula, are 
most complete in young embryos, and are, on the contrary, 
so diminished in subsequent periods, that their functions must 
be connected with the earlier stages of existence. 
The first trace of the formation of an embryo cannot be 
discovered in the different species of this class until a con¬ 
siderable time after conception. The original formation, as 
in the human subject, is widely distant from the subsequent 
perfection of the mature foetus: and the growth and forma- 
tion of the members, instead of proceeding alike in the whole 
class, are so ordered in particular species, that those external 
organs, which are most necessary to the young animal, ac¬ 
cording to its peculiar mode of life, are formed and com¬ 
pleted the soonest. Hence arises the great size of the poste¬ 
rior hands of the fcetal quadrumana, of the feet of the squirrel, 
of such animals in short as are destined to live in trees; like¬ 
wise of those of the foal and kid, which are obliged to use 
their legs immediately after birth, when compared with the 
corresponding parts of the mature human foetus. 
Having gone through the descriptions of the particular 
organs in the five classes, the reader may now turn to the 
following figures, which shew these parts in situ. 
Fig. 3 and 4, already in great part described, are a speci¬ 
men of the internal organs of the class vermes. 
Fig. 4. The dotted portion, i i, of fig. 4, is filled up 
by the muscles that retain or propel the animal, and move 
the intestines; these latter parts and the liver cover the 
shadowed parts. The veins are shewn at a, coming from 
the limbus, and toward the auricle, d, opening into the 
ventricle, c. We see the calcareous sac at e, and its 
duct running along the intestinum rectum, m; h marks 
the aorta arising from the ventricle, and ramifying through 
the body, and g, a remarkable swelling its commencement 
undergoes, similar to what is seen in fishes; o shews the ovi¬ 
ducts,/) the ovaries, and r the oesophagus. 
Fig. 29 is a specimen of the internal organs in the class 
insectae : a a, the two principal trunks of the tracheae, which 
carry the air over the body; h, the branches they send to the 
head; c c, those to the muscles of the thorax, and of the 
wings; d d, those to the abdominal muscles, and to the 
medulla spinalis; e e, to the vesiculae seminales; f, to the 
stigmata; gg, to the skin; i i, to the wing covers; k k, the 
medulla spinalis; * * the optic nerves. 
Fig, 26. A specimen of the internal parts in fishes. An 
internal 
