COMPARATIVE ANATOMY. 



dous mollusca. Here, therefore, impreg- 

 nation is effected out of the body; as it is 

 also in the frog 1 and toad. But in the lat- 

 ter instances the male embraces the fe- 

 male, and discharges his semen in pro- 

 portion as she voids the eggs. In most 

 animals the seminal liqour is introduced 

 into the body of the female, and the ova 

 are fecundated before they are discharg- 

 ed. This is the case in the mammalia, 

 birds, most reptiles, and some fishes ; in 

 the hermaphrodite gastcropodous mol- 

 lusca, in the Crustacea, and insects. The 

 act by which this is accomplished is 

 termed copulation. 



In all the last mentioned orders ova 

 may be discharged without previous co- 

 pulation, as in the preceding ones. But 

 they receive no further developement ; 

 nor can they be fecundated when thus 

 voided. 



The effect of a single copulation va- 

 ries in its degree ; it usually fecundates 

 one generation only ; but sometimes, as 

 in poultry, several eggs are fecundated ; 

 still, however, they only form one gene- 

 ration. 



In a very few instances one act of co- 

 pulation fecundates several generations, 

 which can pcopagate their species with- 

 out 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 

 nourishes 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 number and solidity. The 

 germ, together with its mass of nourish- 

 ment, and the surrounding membranes, 

 constitutes an egg or ovum ; and the ani- 

 mals 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 necessary 

 that it should be afterwards fecundated, 

 as in many fishes, or required only the ap- 

 plication of artificial heat for its incuba- 

 tion, as in birds ; or that the natural heat 

 of the climate is sufficient, as in reptiles, 

 insects, &.c. These are strictly oviparous 

 animals. 



The ovum, after being fecundated, and 

 detached from the ovarium, remains, in 

 some animals, witbin the body of the mo- 



ther, until the contained germ be die. 

 veloped and hatched. These are false 

 viviparous animals, or ovo-viviparous. 

 The viper and some fishes afford instan- 

 ces of this process. 



Mammalia alone are truly viviparous 

 animals. Their germ possesses no pro- 

 vision of nourishment, but grows by what 

 it derives from the juices of the mother. 

 For this purpose it is attached to the in- 

 ternal surface of the uterus, and some- 

 times by accident to other parts, by a 

 kind^ of root or infinite ramification of 

 vessels called a placenta. It is not, 

 therefore, completely separated 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, 

 generation may be said to consist of four 

 functions, differing in their importance, 

 and in the number of animals to which 

 they belong. 



1st. The production of the germ, which 

 is a constant circumstance ; 2dly, fecun- 

 dation, which belongs to only the sexual 

 generations ; 3dly, copulation, which is 

 confined to those sexual generations, in 

 which fecundation is accomplished with- 

 in the body. 



Lastly, uterogestation, which belongs 

 exclusively to viviparous generation. 



The testes, and sometimes the vesicu- 

 Ix seminales and prostate, vary most re- 

 markably in their magnitude in such ani- 

 mals as have a regular rutting season. 

 They are very diminutive at other peri- 

 ods of the year, but swell at that par- 

 ticular time to a comparatively vast mag- 

 nitude. This change is particularly ob- 

 servable in the testes of the mole, spar- 

 row, and frog. 



We may mention here, in a cursory 

 and general manner, the peculiar organs 

 possessed by the moles 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 ornithorhynchus ; the rough 

 black tubercle formed in the spring sea- 

 son on the thumb of the common frog ; 

 the two members, formed of bones arti- 

 culated to each other, near the genitals 

 of the male torpedo and other cartilagi- 

 nous fishes ; the forceps on the abdomen 

 of the male dragon-fly, &c. 



A scrotum belongs to the mammalia on- 

 ly ; and is not found in all these. The 

 aquatic genera, those which live under 

 ground, and others, want it. 



