196 GENERAL PRINCIPLES OF ZOOLOGY. 



animal from an animal of the same or similar structure, 

 can take place either by the sexual or the asexual mode. 



5. Asexual generation is, in the last analysis, caused 

 by the growth of an organism above its individual mass, 

 since this surplus finally leads to a division into two or 

 more individuals. 



6. Asexual generation may be either by division or by 

 budding. 



7. In case of division, an organism grows regularly in 

 all its parts, and by constriction falls into two or more 

 equivalent new pieces. 



8. According to the direction of the plane of division 

 in reference to the long axis of the animal, we speak of 

 longitudinal, transverse, and oblique division. 



9. In case of budding, a local growth occurs; the local 

 outgrowth, the bud, separates itself off from the mother as 

 a smaller, usually also as an incompletely formed, animal. 



10. According to the position and number of the buds, 

 we distinguish lateral, terminal, and multiple budding. 



11. Sexual reproduction is reproduction by means of 

 more special cells, the sexual cells, which do not take part 

 in the ordinary growth of the body. 



12. In sexual reproduction two kinds of cells unite, the 

 female egg and the male spermatozoon (fertilization). 



13. In rare cases the egg develops without fertilization : 

 parthenogenesis ; this is a sexual reproduction with degen- 

 erated fertilization. 



14. Padogenesis is parthenogenetic reproduction by a 

 young (i.e., incompletely developed) animal. 



15. Different modes of reproduction (asexual, sexual, 

 parthenogenesis, paedogenesis) may occur in the same spe- 

 cies; then these often occur in a regular order, and in 

 such a way that individuals with different modes of repro- 

 duction alternate with one another: alternation of genera- 

 tions in the wider sense. 



1 6. Alternation of generations in the narrower sense 

 (progessive generation, metagenesis] is the alternation of 

 two generations, of which one reproduces by division or 



