COMPARATIVE ANATOMY. 
required for their further developement. 
These operations constitute fecundation, 
and suppose the existence of sexual parts ; 
•which may either be sepai'ate or united in 
the same individual. 
The office of the male sex is that of fur- 
nishing the fecundating or seminal fluid ; 
but the manner in which that contributes 
to the developement of the germ, is not yet 
settled by physiologists. Some, forming 
their opinions from the human subject and 
the mammalia, where the germs are imper- 
ceptible before fecundation, suppose that 
these are created by the mixture of the 
male fluid with that which they suppose to 
exist in the female ; or that they pre-exist 
in the male semen, and that the female only 
furnishes them with an abode. Others con- 
sult the analogy of the other classes of ani- 
mals and of plants. In several instances, 
particularly in the frog, the germ may be 
clearly recognised in the ovum, before fe- 
cundation : its pre-existence may be con- 
cluded in other cases from the manner in 
which it is connected to the ovum when it 
first becomes visible ; for it is agreed on all 
sides that the ovum exists in the female 
before fecundation, since virgin hens 
lay eggs, &c. From such considerations 
these physiologists conclude, that the germ 
pre-exists in all females, and that the fecun- 
dating liquor ' is a stimulus which bestows 
on it an independent life, by awakening it, 
in a manner, from the species of lethargy 
in which it would otlierwise have constantly 
remained. 
The origin of the germs, and the mode of 
their existence in the female, whether they 
are formed anew by the action of life, or 
are pre-existent, and inclosed within each 
other; or whether they are disseminated, 
and require a concourse of circumstances 
to bring them into a situation favourable 
for their developement, are questions which, 
in the present state of our knowledge, it is 
utterly impossible for us to decide. These 
points have for a long time been agitated 
by physiologists ; but the discussion seems 
now to be abandoned by universal consent. 
The combination of the sexes, and the 
mode of fecundation are. subject to great 
variety. In some instances they are united 
in the same individual, and the animal im- 
pregnates itself. The acephalous mollusca 
and the echinus exemplify this structure. 
In others, although the sexes are united in 
each individual, an act of copulation is re- 
quired, in which they both fecundate and 
are fecundated. This is the case with the 
gasteropodous mollusca, and several wormsi 
In the remainder of the animal kingdom 
the sexes belong to different individuals. 
The fecundating liquor is always applied 
upon or about the germs. In many cases 
the ova are laid before they are touched by 
the semen ; as in some fishes of the bony 
division, and the cephalopodous mollusca. 
Here, therefore, impregnation is effected 
out of the body ; as it is also in the frog 
and toad. But in the latter instances the 
male embraces the female, and discharges 
his semen in proportion as she voids the 
eggs. In most animals the seminal liquor 
is introduced into the body of the female, 
and the ova are fecundated before they are 
discharged. This is the case in the mam- 
malia, birds, most reptiles, and some 
fishes ; in the hermaphrodite gasteropodous 
mollusca, in the Crustacea andinsects. The 
act by which this is accomplished, is termed 
copulation. 
In all the last mentioned orders ova may 
be discharged without previous copulation, 
as in the preceding ones. But they re- 
ceive no further developement ; nor can 
they be fecundated when thus voided. 
The effect of a single copulation varies in 
its degree ; it usually fecundates one gene- 
ration only ; but sometimes, as in poultry, 
several eggs are fecundated ; still, how- 
ever, they only form one generation. 
In a very few instances one act of copu- 
lation fecundates several generations, which 
can propagate their species without the aid 
of the male. In the plant-louse (aphis) 
this has been repeated eight times ; and 
in some monoculi twelve or fifteen times. 
When the germ is detached from the 
ovary, its mode of existence may be more 
or less complete. In most animals it is 
connected, by means of vessels, to an or- 
ganised mass, the absorption of which nou- 
rishes and developes it until the period of 
its birth. It derives nothing, therefore, 
from the body of the mother, from which 
it is separated by coverings varying in num- 
ber and solidity. The germ, together W'ith 
its mass of nourishment, and the surrounding 
membranes, constitutes an egg or ovum ; 
and the animals which produce their young 
in this state, are denominated oviparous. 
In most of these the germ contained in 
the egg is not developed until that part has 
quitted the body of the mother, or has been 
laid ; whether it be necessai'y that it should 
be afterwards fecundated, as in many 
fishes, or require only the application of ar- 
tificial heat for its incubation, as in birds 
