152 GKXERAL PRlXCirLKS OF ZOOLOGY 



6. In division an or!j;anisn"i grows rcgularh' in all its parts, and by 

 constriction falls iiuo two or more equi\'alcnl now ]>icccs. 



7. Accortling to the direction of the plane of division in reference to 

 the long axis of the animal we speak of longitudinal, Iransversc, and 

 oblique division. 



S. In case of biidt!i)!g a local growth occurs ; the local outgrowth, the 1 )ud, 

 separates from the mother as a smaller, usually incompletely formed 

 animal. 



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

 lateral, terminal, and multiple budding. 



10. Sexual rfprch/iniioit occurs In' means of special sexual cells, which 

 have no part in the ordinary functions of the body. 



11. In se.xual 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 tlegenerated fertilization. 



i:;. PicJogenesis is parthcnogenetic reproduction bv a voung (i.e., 

 incompletely developed) animal. 



14. Ditlerent modes of reproduction (asexual, sexual, parthenogciiic, 

 paxiogenic) mav occur in the same species; thou these often occur in a 

 regular order, antl in sucli a way that individuals with ilitTerent modes 

 of reproduction alternate with one another: allernalion of generations in 

 the wider sense. 



15. Allernalion of ge)ierations 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. 



16. The alternation of parthenogenesis or pa~dogenesis with pro- 

 nounced sexual reproduction is called Inierogonv. 



17. Development which is inaugurated b)' sexual re]iroduction shows 

 in nearly all multicellular animals a general agreement in the incipient 

 stages: fertilization, cleavage, formation of germda.vers. 



18. The essential ]Kiint 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. 



ig. The eleai-age oj the egg is a cell di\ision, a division of the fertilized 

 egg into the clea\-age spheres (bl'ist(nneres). The cleavage mav be 

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

 cqnal or nneipial, the partial either diseoidal or siiperjieial. 



20. Ry repeated (li\isions of the cleavage spheres, and by the forma- 

 tion of a cleavage ea\ity, there arises a one-layere^l embryo, the hlaslnla. 



