162 GENERAL PRINCIPLES OF ZOOLOGY. 



systematic importance. In many divisions of animals oviparous as well 

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

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

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

 surf perches, EmbiotocidaB, of the Pacific coast and many Cypriuodouts 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 Ornitho- 

 rhynchus 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. 



SUMMARY OF THE FACTS OF ONTOGENY. 



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

 tion; spontaneous generation and generation by parents are to be 

 distinguished. 



2. Spontaneous generation (generatio aequivoca, or spontanea; 

 abiogenesis) is the origin of living beings from lifeless matter 

 (without pre-existing organisms). 



3. The present 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 by parents (tocogony), derivation of an animal 

 frem 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 two or more equivalent new 

 pieces. 



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

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

 transverse, and oblique division. 



8. In case of budding 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 multiple budding. 



10. Sexual reproduction is reproduction by means of special 

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

 the body. 



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

 egg and the male spermatozoon (fertilization). 



