See also reproduction. It was thought that everything develops from an egg. However there are exceptions to this rule, and outline as briefly as possible the various methods of reproduction which are known to occur among animals.
Reproduction is the formation of new individuals by division ; this is frequently preceded by conjugation (in the protozoa) or fertilization (in both the protozoa and the metazoa). See also cell division.
Three principal methods of reproduction occur in the protozoa.
(1) Binary fission appears to be the most primitive. The individual divides into two parts which are similar in size and structure ; these grow into cells like the original parent. Many CILIATA, FLAGELLATA, and RHIZOPODA normally reproduce in this way.
(2) Budding occurs when a small outgrowth or bud separates from the parent cell. This method occurs among the SUCTORIA, RADIOLARIA, HELIOZOA, CILIATA, and MYXOSPORIDIA.
(3) Sporulation results from the division of the nucleus of the parent into many daughter nuclei and a subsequent division of the cell into as many "spores" as there are nuclei. This process is characteristic of the SPOROZOA and also is found among the RHIZOPODA.
Conjugation is of frequent occurrence in the PROTOZOA. Two or more indi viduals may become connected without fusion of nuclei or cytoplasm, thus forming colonies ; a pair of individuals may unite either temporarily or permanently 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, flatworms, segmented round-worms, and bryozoans. Even the tunicates, which occupy an advanced position 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 fertilized ; that is, they are parthenogenetic. In rare cases such parthenogenetic eggs may be produced, as in the fly Miastor, by immature individuals. When this occurs, reproduction is said to be pcedogenetic.
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.
