152 GENERAL PRINCIPLES OF ZOOLOGY 



6. In 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 reference 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 outgrowth, the bud, 

 separates from the mother as a smaller, usually incompletely formed 

 animal. 



9. According to the position and number of the buds we distinguish 

 lateral, terminal, and multiple budding. 



10. Sexual reproduction occurs by means of special sexual cells, which 

 have no 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). 



12. In rare 'cases the egg develops without fertilization: partheno- 

 genesis; this is a sexual reproduction with degenerated fertilization. 



13. P ado genes is is parthenogenetic reproduction by a young (i.e., 

 incompletely developed) animal. 



14. Different modes of reproduction (asexual, sexual, parthenogenic, 

 predogenic) may occur in the same species; then these often occur in a 

 regular order, and in such a way that individuals with different modes 

 of reproduction alternate with one another: alternation of generations in 

 the wider sense. 



15. Alternation of generations in the strict sense (metagenesis} is the 

 alternation of two generations, one reproducing by division or budding, 

 the other sexually. The former is called the nurse, the latter the sexual 

 animal. 



1 6. The alternation of parthenogenesis or pasdogenesis with pro- 

 nounced sexual reproduction is called heterogony. 



17. Development which is inaugurated by sexual reproduction shows 

 in nearly all multicellular animals a general agreement in the incipient 

 stages: fertilization, cleavage, formation of germ-layers. 



18. The essential point of fertilization lies in the complete fusion of 

 egg and spermatozoon, particularly in the fusion of the nuclei, egg and 

 sperm nuclei, to form the cleavage nucleus. 



19. The cleavage of the egg is a cell division, a division of the fertilized 

 egg into the cleavage spheres (blastomeres). The cleavage may be 

 total (holoblastic egg) or partial (meroblastic egg) ; total cleavage is either 

 equal or unequal, the partial either discoidal or superficial. 



20. By repeated divisions of the cleavage spheres, and by the forma- 

 tion of a cleavage cavity, there arises a one-layered embryo, the blastula. 



