METAZOA. 71 



the ordinary form of the species unless it first conjugates with the 

 spermatozoon. The zygote so produced is uninuclear, and has the 

 property of developing into the ordinary form of the species. This 

 method of reproduction, in which a new individual arises from the 

 combination of two independent individualities in the zygote, is 

 called the sexual method of reproduction, as opposed to the asexual 

 method, in which a multinucleated mass is separated off from the 

 parent with the power of assuming more or less directly the ordinary 

 form of the species. The asexual method, though common in the 

 vegetable kingdom, is comparatively rarely found amongst animals 

 (Coelenterata, Polyzoa, Tunicata, Annelida, etc.). 



It thus appears that the Metazoa may be defined as 

 Animals in which the ordinary (so-called adult) form of the species 

 has always more than one nucleus, and in which the nuclei are for 

 the most part arranged regularly and with a definite relation to the 

 functional tissues of the animal (so-called cellular arrangement). 

 Special conjugating individuals of the form of ova and spermatozoa 

 are always formed. 



