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. 
Merazoa 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. Asa 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 fertilized; 
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 
pedogenetic. 
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 diecious and the 
individuals gonochoristic. In many species there is 
but a single sort of individual which produces both 
eggs and spermatozoa; such species are monecious, 
and the individuals are hermaphroditic. 
