COMPARATIVE ANATOMY, 
er that the natural heat of the climate is 
sufficient, sis in reptiles, insects, &c. These 
are strictly oviparous animals. 
'I'he ovum, after being fecundated, and 
detached from the ovarium, remains in 
some animals within the body of the mother, 
until the contained germ be developed and 
hatched. These are false viviparous ani- 
mals, or ovo-viviparous. The viper and 
some fishes afford instances of this process. 
Mammalia alone are truly viviparous 
smimals. Their germ possesses no provi- 
sion of nourishment, but grows by what it 
derives from the juices of the mother. 
For this purpose it is attached to the inter- 
nal surface of the uterus, and sometimes by 
accident to other parts, by a kind of root or 
infinite ramification of vessels called a pla- 
centa. It is not, therefore, completely se- 
parated from the mother by its coverings. 
It does not come into the world until it can 
enjoy an independent organic existence. 
The mammalia cannot, therefore, be said to 
possess an ovum in the sense which we have 
assigned to that term. 
From the above view of the subject, ge- 
neration may be said to consist of four 
fiinctions, differing in their importance, and 
in the number of animals to which they be- 
long. 
1st. The production of the germ, which 
is a constant circumstance ; 2dly, fecunda- 
tion, which belongs only to the sexual gene- 
ration j 3dly, copulation, which is confined 
to those sexual generations, in which fecun- 
dation is accomplished within the body. 
Lastly, uterogestation, which belongs ex- 
clusively to viviparous generation. 
The testes, and sometimes the vesicul® 
seminales and prostate vai-y most remark- 
ably in their magnitude in such animals as 
have a regular rutting season. They are 
very diminutive at other periods of theyear, 
but swell at that particular time to a com- 
paratively vast magnitude. This change is 
particulai'ly observable in the testes of the 
mole, sparrow, and frog. 
We may mention here, in a cursory and 
general manner, the peculiar organs pos- 
sessed by the males of some species for the 
purpose of holding the female during the 
act of copulation. Of this kind are, the 
spur on the hind-feet of the male omi- 
thorhynchus ; the rough black tubercle 
formed in the spring season on the thumb of 
the common frog; the two members, 
formed of bones articulated to each other, 
near the genitals of the male torpedo and 
other cartilaginous fishes ; the forceps oa 
the abdomen of the male dragon-fly, &c. 
A scrotum belongs to the mammalia 
only ; and is not found in all these. The 
aquatic genera, those which live under 
ground, and others want it. 
The testes remain constantly in the ab- 
domen in the omithorhynchus, the ele- 
phant, the amphibious mammalia, and the 
cetacea. Some animals have the power of 
protruding them from the abdomen, and re- 
tracting them again into the cavity ; as the 
bats, mole, hedgehog, and shrew, besides 
several of the rodentia. They are thrust 
out of the cavity, particularly at the rutting 
season. 
The tunica vaginalis exists constantly in 
the mammalia. As the horizontal position of 
the body obviates the danger of herni®, the 
cavity of this membrane communicates by 
means of a narrow canal with the abdomen, 
in such animals as have the testes remaining 
constantly in the scrotum. 
In some species, where the act of copu- 
lation requires a longer portion of time, as 
in the dog, badger, &c. the corpus spon- 
giosum of the glans, and of the posterior 
part of the penis, swells during the act 
much more considerably than the rest of the 
organ, and thus the mate and female are 
held together during a sufficient space of 
time for the discharge ,of the seminal 
fluid. 
Several species of mammalia, both among 
those which possess no vesicul® seminales, 
and thereby require a longer time for com- 
pleting the act of copulation, and in such 
as are not distinguished by this peculiarity, 
possess a peculiar bone in the penis, gene- 
rally of a cylindrical form, but sometimes 
grooved. This is the case with some of the 
simi®, most of the bat-kind, the hamster 
and several others of the mouse-kind, the 
dog, bear, badger, weasel, seal, walrus, 
&c. 
In most of the mammalia the urethra 
rims on to the end of the glans, and forms a 
common passage for the urine, prostatic li- 
quor and semen. In some few species, the 
passage which conducts the two former 
fluids, is distinct from tliat of the seminal 
liquor. The bifid fork-like glans of the 
opossum has three openings, one at the 
point of bifurcation for transmitting the 
urine ; and two for the seminal fluid at the 
two extremities of the glans. The short 
urethra of the omithorhynchus paradoxus 
opens directly into the cloaca, and the 
large penis of the animal serves merely to 
