126 REPRODnCTION IN PLANTS AND ANIMALS. 



them. The organs in which these germs are developed, 

 are called female sexual organs ; the organs which form 

 the matter which fecundates them, are called male sexual 

 organs. 



That all animals are produced from eggs, omne vimtm 

 ex ovo, is an old adage which modem researches have 

 abundantly confirmed. In tracing back the phases of ani- 

 mal life, we invariably arrive at an epoch when the inci- 

 pient animal was enclosed in an egg. It is then called an 

 embryo, and the period passed in this condition, which is 

 more or less long according to the nature of t'he animal, 

 is called the embryonic period. 



Before the various classes of the animal kingdom had 

 been attentively compared during the embryonic period, all 

 animals were divided into two great divisions ; the ovipar- 

 ous, comprising those which lay eggs, such as birds, rep- 

 tiles, insects, and moUusks; and the viviparous, which 

 bring forth their young alive, viz., the mammalia. This dis- 

 tinction has, however, lost much of its importance ; for it has 

 been ascertained that viviparous animals are produced from 

 eggs as well as the oviparous, and that both have one and 

 the same uniform structure in the beginning. In vivipar- 

 ous animals, however, the eggs undergo their early changes 

 in the body of the mother, that is to say, the embryo de- 

 velops in the uterus, and bursts the membrane of the egg, 

 which it leaves there, coming out of the body of the mother 

 naked and already formed ; whereas in oviparous animals, 

 the egg is laid with its membranes and the germ which it 

 contains, and the development of the embryo is extra- 

 uterine. 



Production from eggs must therefore be considered as a 

 universal characteristic of the animal kingdom. Even 

 animals which propagate by gemmiparous and fissiparous 



