18 GERM-CELL CYCLE IN ANIMALS 



manently with fusion of the cytoplasm only; or 

 both cytoplasm and nuclei of such a pair may fuse 

 or be interchanged. 



Metazoa reproduce either sexually or asexually. 

 Asexual reproduction is reproduction without the aid 

 of sex cells. It takes place as a rule by means of 

 buds or by fission as in many polyps, sponges, 

 flat- worms, segmented round-worms, and bryozoans. 

 Even the tunicates, which occupy an advanced posi- 

 tion in the animal series, form buds. Some of the 

 sponges produce internal buds called gemmules, 

 and certain bryozoans form similar bodies known 

 as statoblasts. Sexual reproduction requires that 

 the individual develop from a mature egg. As a rule 

 the egg must be fertilized by the union with it of a 

 spermatozoon, thus forming a zygote ; but the eggs 

 .of many animals develop without being fertihzed ; 

 that is, they are parthenogenetic. In rare cases such 

 parthenogenetic eggs may be produced, as in the 

 fly Miastor (see Chapter III), by immature individ- 

 uals. When this occurs, reproduction is said to be 

 poedogenetic. 



The sex of an animal is judged by the kind of sex 

 cells it produces, — eggs by the female and sperma- 

 tozoa by the male, — and when the individuals of a 

 single species are differentiated as either males or 

 females, the species is said to be dioecious and the 

 individuals gonochoristic. In many species there is 

 but a single sort of individual which produces both 

 eggs and spermatozoa ; such species are monoecious, 

 and the individuals are hermaphroditic. 



