GENERAL EMBRYOLOGY. 161 



sexually mature animal. In indirect development, on the con- 

 trary, organs belonging only to the larval life, and hence called 

 larval organs, are formed and later are destroyed. Therefore in 

 the definition of indirect development, or as it is commonly called 

 metamorphosis, special emphasis is 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 distinguished from frogs not only by 

 the absence of lungs and extremities, but also by the presence of 

 gills and tail. The more numerous the larval organs, the more 

 pronounced, therefore, will be the metamorphosis. 



Oviparous and Viviparous Animals. The time at which the 

 egg leaves the mother's body is independent of that 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. Only those forms can be 

 considered as strictly oviparous in which the egg at the time of 

 laying is 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 batrachians and insects. In viviparous 

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

 branes occur quite, or almost, at the same time, and from the 

 mother there emerges an animal which has completed its develop- 

 ment or, at least, has progressed so far that it is able to live with- 

 out protective coverings. 



Ovo-viviparous Animals. Varying degrees of ovo-viviparous develop- 

 ment connect these two extremes. 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 artificial rupture 

 of the egg membranes, an embryo more or less developed, but usually not 

 yet capable of independent life, is exposed. Birds really belong in the 

 category of ovo-viviparous animals, for their eggs are fertilized some time 

 before they are laid, and have already completed the formation of the 

 blastoderm. In the case of many worms the egg-shell may contain, even 

 at the time of laying, an animal all ready for hatching. 



No Sharp Line between Oviparous and Viviparous. Transitional forms 

 of this kind show that no sharp line can be drawn between * egg-laying ' 

 and ' bearing living young ' and one must guard against attributing too 

 much importance to the apparent distinctions. Linnaeus, following the 

 example of Aristotle, was in error in regarding the time of birth as of 



