REPRODUCTION. 419 



impregnation is internal, and their subsequent development 

 takes place in one or other of the four following ways. 



1. The ovum, when defended by a firm envelope, which 

 contains a store of nutriment, is termed an egg> and is de- 

 posited in situations most favourable for the development of 

 the embryo; and also for its future support when it emerges 

 from the egg. Birds, as is well known, produce eggs which 

 are incased in a calcareous shell, and hatch them by the 

 warmth they communicate by sitting on them with unwea- 

 ried constancy. All animals which thus lay eggs are termed 

 oviparous. 



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

 lamander, 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 ani- 

 mals are said to be Ovo-viviparous. There are other tribes, 

 again, which, according to circumstances, are either ovipa- 

 rous, or ovo-viviparous: this is the case with the Shark. 



3. Viviparous animals are those in which no egg, pro- 

 perly so called, is formed; but the ovum, after proceeding 

 through the oviduct, sends out vessels, which form an at- 

 tachment to the interior of a cavity in the body of the pa- 

 rent, whence it draws nourishment, and therefore has at- 

 tained a considerable size at the time of its birth. 



4. Marsupial animals are those, which, like the Kangu- 

 roo, and the Opossurn, are provided with abdominal pouches, 

 into which the young, born at a very early stage of develop- 

 ment, are received, and nourished with milk, secreted from 

 glands contained within these pouches. As the young, both 

 in this and in the last case, are nourished with milk prepared 

 by similar glands, or Mammse, the whole class of vivipa- 

 rous and marsupial animals has received, from this charac- 

 teristic circumstance, the name of Mammalia. 



