102 GENERAL PRINCIPLES OF ZOOLOGY. 



systematic importance. In many divisions of animals oviparous as well 

 as viviparous forms are found. Tiie majority of sharks are viviparous, 

 but a few species lay eggs ; on the contrary, for Vjony fishes the rule holds 

 that the eggs are laid before fertilization. Exceptions are tlie viviparous 

 surf perches, Embiotocidse, of the Pacific coast and many Cyprinodonts of 

 fresh water. Most of the Amphibia, reptiles, and insects are egg-layers, 

 but not a few forms are viviparous. Even among the mammals, for 

 which for a long time the ' bearing young alive ' was regarded as diag- 

 nostic, it has been discovered lately that the Echidna and Ornitlio- 

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

 same species. Adders commonly lay eggs, but under unfavorable condi- 

 tions they retain them inside their body until ready to hatch. 



SUM3IARY OF TIIE FACTS OF OXTOriFXY. 



1. The derelojoment of an animal begins with an act of genera- 

 tion; spontaneous generation and generation by parents are to be 

 distinguished. 



2. Spontaneotis generation (generatio fequivoca, or spontanea; 

 abiogenesis) is the origin of living beings from lifeless matter 

 (without pre-existing organisms). 



3. The i^reseut existence of spontaneous generation is neither 

 shown by observation, nor is it, on the whole, probable ; yet spon- 

 taneous generation is a logical postulate, in order to explain the 

 first origin of life on our globe. 



■4. Generation ly jiarents (Tocogony), derivation of an animal 

 from an animal of the same or similar structure, can take place 

 either by the sexual or the asexual mode. 



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



6. In case of division an organism grows regularly in all its 

 parts, and by constriction falls into tw-o or more equivalent new 

 pieces. 



7. According to the direction of the plane of division in refer- 

 ence to the long axis of tlie animal we speak of longitudinal, 

 transverse, and oblique division. 



8. In case of ludding a local growth occurs: the local out- 

 growth, the bud, separates from the mother as a smaller, usually 

 incompletely formed, animal. 



9. According to the position and number of the buds we dis- 

 tinguish lateral, terminal, and multii)le buddino-. 



10. Hexual reproduction is re2)roduction Ijy means of special 

 sexual colls, which do not take part in the ordinary functions of 

 the body. 



11. In sexual rejiroduction two kinds of cells unite, the female 

 egg and the nnile spermatozoon (fcu'tilization). 



