422 THILOSOPHY 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, in many cases, inde- 

 pendent of their parent, and do not stand in need of any 

 * assistance : they are born in 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 branchiae ; both of 

 which are absorbed, and disappear, when it becomes an 

 inhabitant of the land. The infancy 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 suffering a transformation of parts 

 previous to appearing 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 eggs during the period 



