429, FHILOSOPHY OF ZOOLOGY. 
the Rana pipa, the eggs are deposited in a bag on the 
back, where they are hatched, and where the young ani- 
mals reside for some time after birth. Some animals, as 
the Aphis, are oviparous at one season, and ovoviviparous 
at another. 
In what manner the blood of the embryo of ovoviviparous 
animals is aerated, has not been satisfactorily determined. 
The young, after being hatched, are, m many Gases, inde- 
pendent of their parent, and do not stand in need of any 
assistance: they are born m the midst of plenty, and have 
organs adapted to the supply of their wants. ‘Thus, many 
insects are hatched on, or within the very leaves which 
they are afterwards to devour. In other cases, the young 
are able to follow their parents, and receive from them a 
supply of appropriate food; or if unable to follow, their 
parents bring their food to the nests. 
The changes which the young of oviparous animals un- 
dergo in passing from infancy to maturity, have long at- 
tracted the notice of the inquisitive observer. ‘The egg of 
the frog is hatched in the water, and the young animal 
spends in that element a part of its youth. While there, 
it is furnished with a tail and external branchiee; both of 
which are absorbed, and disappear, when it becomes an 
inhabitant of the land. The mfancy of the butterfly is 
spent in the caterpillar-state, with organs of motion and 
mastication which are peculiar to that period. It is de- 
stined to endure a second hatching, by becoming envelop- 
ed in a covering, and suffermg a transformation of parts 
previous to appearmg in its state of maturity. These meta- 
morphoses of oviparous animals present an almost infinite 
variety of degrees of change, differing in character accord- 
ing to the tribes or genera. 
In birds, it is well known that one sexual union suffices 
for the production of impregnated eges during the period 
