80 COLLEGE ZOOLOGY 



this occurs, the term parthenogenesis is applied to the process. 

 For example, certain eggs of plant lice (Aphids) and water fleas 

 (Daphnia) normally develop parthenogenetically. In a few cases 

 animals which have not reached maturity produce eggs which 

 develop without being fertilized; this sort of parthenogenesis 

 is called pedogenesis. For example, the larvae of a gall-gnat, 

 and the pupae of a midge, produce eggs which develop without 

 fertilization. 



A species of animal in which each individual possesses only 

 one kind of reproductive organs, either male or female, is 

 dioecious. A species with both male and female reproductive 

 organs in the same individual is monoecious, or hermaphroditic. 

 Hydra (Figs. 65-72) and the earthworm (Figs. 153-159) are 

 examples of monoecious animals; the crayfish (Figs. 200-208) 

 is a dioecious species. 



If the eggs of a monoecious animal are fertilized by the same 

 individual, self-fertilization occurs; whereas, if the egg of one 

 individual unites with the spermatozoon of another, cross- 

 fertilization results. 



Animals which lay eggs, like a bird or crayfish, are oviparous; 

 those which bring forth young from eggs developed within the 

 body, like mammals and certain snakes, viviparous. 



Asexual Reproduction. — This term is applied to reproduc- 

 tion by means of budding or fission, and not by the production 

 of eggs. By fission is meant the division of the parent into 

 two or more equivalent parts, the daughters. This occurs 

 frequently in Protozoa (Ameba, p. 32, Paramecium, p. 59, 

 Euglena, p. 44), and less often in Metazoa. The fresh-water 

 flatworm, Planaria (Figs. 97-102), and the annelid, Dero, often 

 divide by fission. The offspring produced by budding are smaller 

 than their parent. Hydra (Fig. 65) affords an excellent example 

 of an organism that reproduces in this way. 



Metagenesis. — Some animals reproduce by budding and 

 do not develop eggs nor spermatozoa. Certain of the buds, 

 however, separate from the parent and produce reproductive 



