194 GENERAL PRINCIPLES OF ZOOLOGY. 



it is commonly called metamorphosis, special emphasis is 

 to be laid upon the presence of "larval organs." Thus 

 the caterpillars of butterflies are distinguished, not only by 

 the absence of compound eyes and wings, but also by the 

 presence of anal feet and spinning-glands, which are absent 

 in the butterfly, and further by the different shape of the 

 jaws, antennae, and legs, the different arrangement of the 

 tracheae and nervous system, etc. Tadpoles are distin- 

 guished from frogs, not only by the absence of lungs and 

 extremities, but also by the presence of gills and tail. The 

 more evident the larval organs, the plainer, therefore, will 

 be the character of the metamorphosis. 



Oviparous and Viviparous Animals. The point of 

 time at which the egg leaves the mother's body is inde- 

 pendent of the time at which the embryo escapes from the 

 egg-membranes. Two extremes are known, the oviparous 

 or egg-laying animals, and the viviparous or those which give 

 birth to living young. As strictly oviparous animals can be 

 considered only those forms in which the egg, at the time 

 of laying, has still the character of a single cell, in which case 

 it is either not fertilized until after extrusion, as in the case 

 of most fishes, sea-urchins, etc., or during extrusion, as in the 

 case of batrachians and insects. In the case of viviparous 

 animals, on the contrary, birth and the rupture of the egg- 

 membranes occur quite, or almost, at the same time, and 

 from the maternal vagina there emerges an animal which 

 has completed its development or, at least, has progressed 

 so far that it is able to live without protective coverings. 



Ovo-viviparous Animals. Varying degrees of " ovo- 

 viviparous'''' development mediate between these two ex- 

 tremes. What here appears at birth, at first impresses us 

 on account of its covering as being an egg; but the first 

 stages of development have already passed, so that, by arti- 

 ficial rupture of the egg-membranes, an embryo more or 

 less developed, but usually not yet capable of independent 

 life, is exposed. Birds are to be reckoned in the category 

 of ovo-viviparous animals, for their eggs are fertilized long 

 before they are laid, and have already completed the for- 



