154 



GENERAL ZOOLOGY 



function as an ovary, at another time as a testis, a condition 

 known as dichogamy. Ova or eggs are produced in the ovary 

 and spermatozoa or sperm cells in the testis. Ova and sperma- 

 tozoa are spoken of generally as germ cells or gametes. In the 

 method of reproduction known as amphigony, a spermatozoon 

 (in some cases several) enters the egg and forms an oosperm or 

 fertilized egg or zygote, which develops into a new individual 

 animal. The details of the process of fertilization vary, but its 

 essential feature is the union of a nucleus derived from the 

 sperm cell with the egg nucleus. If more than one sperm cell 



enters the egg, as for example in 

 the hen's egg, only one sperm 

 nucleus combines with the egg 

 nucleus. Fertilization may take 

 place before or after the egg 

 leaves the body of the female. 

 In parthenogenesis, the egg de- 

 velops without the intervention 

 of the spermatozoon. Some eggs 

 develop either parthenogenetic- 

 ally or in response to fertiliza- 

 tion. Usually an egg requiring 

 fertilization for its development 

 Fig. 99.— a, fission in Hydra by does not develop parthenoge- 

 longitudinai division. (After Koditz.) ne ti ca lly, under normal condi- 



B, fission in Planaria. (After Child.) . ■" . ... 



tions. An oviparous animal is 

 one in which the young are hatched from eggs laid by the female. 

 A viviparous animal gives birth to its young. Both methods 

 of producing young occur in both invertebrates and vertebrates. 

 Agamic Reproduction. — Some animals reproduce by fission, 

 which consists in a division of the entire body into two parts of 

 approximately the same size. Since no gametes or germ cells 

 are involved, fission is an agamic or asexual form of reproduction. 

 Another form of this type of reproduction is budding, which differs 

 from fission in that the process starts with the formation of a 

 bud on the surface of the body. The bud enlarges, differentiates, 

 and finally detaches itself as a small fully formed new individual. 

 Both fission and budding occur in Hydra. Fission takes place 

 by a longitudinal splitting, beginning at the distal end and 

 extending down through the basal disk, or by a transverse con- 



