GENERAL EMBRYOLOGY 151 



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, as in the case of most fishes, 

 sea-urchins, batrachians, insects, etc. In viviparous animals, on the con- 

 trary, birth and the rupture of the egg membranes occur quite, or almost, 

 at the same time, and from the mother 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 develop- 

 ment connect these two extremes. What here appears at birth at first im- 

 presses 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 ci;g 

 membranes, an embryo more or less developed, but usually not yet capable of 

 independent life, is exposed. Birds really belong in the category of ov<>- 

 viviparous animals, for their eggs are fertilized before they are laid, and ha\c 

 already completed the formation of the blastoderm. In the case of many 

 snakes the egg-shell may contain, even at the time of laying, an animal all 

 ready for hatching. Transitional forms of this kind show that no sharp liiu- 

 can be drawn between 'egg-laying' and 'bearing living young' and one must 

 guard against attributing too much importance to the apparent distinctions. 

 In many divisions of animals oviparous as well as viviparous forms are found. 

 The majority of sharks are viviparous, but some lay eggs; in most bony fishes 

 the eggs are laid before fertilization. Exceptions are the viviparous surf perches 

 of the Pacific and many Cryprinodonts of fresh water. Most of the Amphibia, 

 reptiles, and insects are egg-layers, but not a few forms are viviparous and 

 even in the same genus (Lacerta and Chamivlemi) there may be viviparous and 

 ovo-viviparous species. Even among the mammals, for which for a long time 

 the 'bearing young alive' was regarded as diagnostic, it has been discovered that 

 the monotremes lay eggs. Finally, exceptions to the rule occur in one and the 

 same species. Adders are ovo-viviparous, but under unfavorable conditions they 

 retain the eggs inside their body until ready to hatch. 



SUMMARY OF THE FACTS OF ONTOGKXV. 



1. The development of an animal begins with an act of generation; 

 spontaneous generation and generation by parents are to be distinguished. 



2. Spontaneous generation (abiogenesis) is the origin of living beings 

 from lifeless matter (without pre-existing organisms). 



3. The present existence of spontaneous generation is neither known 

 nor is it, on the whole, probable; yet it is a logical postulate, in order to 

 explain the origin of life on our globe. 



4. Generation by parents, derivation of an animal from another of 

 similar structure, can take place either by the sexual or the asexual mode. 



5. Asexual generation may be either by division or by budding. 



