REPRODUCTION. 597 



flowers which contain only one of the kinds of 

 reproductive apparatus: so that the stamens and 

 the pistils are situated on separate plants : and 

 the impregnation of the ovaries in the latter, can 

 be effected only by the transference of the pollen 

 from the former. A similar separation of offices 

 is established among all the higher classes of the 

 animal kingdom. In most Fishes, and in all 

 Batrachian reptiles, the ova are impregnated 

 after their expulsion from the body : in all other 

 cases their impregnation is internal, and their 

 subsequent developement takes place in one or 

 other of the four following ways. 



1 . The ovum, when defended by a firm enve- 

 lope, which contains a store of nutriment, is 

 termed an egg, and is deposited in situations 

 most favourable for the developement of the 

 embryo ; and also for its future support when it 

 emerges from the egg. Birds, as is well known, 

 produce eggs which are encased in a calcareous 

 shell, and hatch them by the warmth they com- 

 municate by sitting on them with unwearied 

 constancy. All animals which thus lay eggs 

 are termed oviparous. 



2. There are a few tribes, such as the Viper 

 and the Salamander, whose eggs are never laid, 

 but are hatched in the interior of the parent ; so 

 that they bring forth living offspring, although 

 originally contained in eggs. Such animals are 

 said to be Oio-viviparous. There are other 



