536 THE REPRODUCTIVE FUNCTIONS. 



its course. In a limited number of plants com- 

 posing the class Dioecia, the individuals of the 

 same species are distinguished by. their bearing 

 flovt^ers 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 Ba- 

 trachian reptiles, the ova are impregnated after 

 their expulsion from the body : in all other cases, 

 their impregnation is internal ; and their subse- 

 quent 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 eg<f, and is deposited in situations most favour- 

 able 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 communicate 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 Vipe?- 

 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 origi- 

 nally contained in eggs. Such animals are said to 

 be Ovo-viviparoiis. There are other tribes, again, 

 which, according to circumstances, are either ovi- 



