INTRODUCTION. XXI 



the two sexes are, in most cases, distinguished by 

 various external peculiarities. 



The independent being which results from the 

 complete development of a fertilised ovum may 

 subsequently multiply itself in various modes 

 which have been grouped together under the 

 common name of "asexual generation." These 

 are all reducible to two processes, namely, * fis- 

 sion,' or the division of the body into separate 

 parts, and 'gemmation,' or the formation of 

 buds. Gremmation may be either internal or 

 external. 'S^Tien the products of the latter remain 

 in connection with the parent organism, we are 

 furnished with an illustration of the tendency to 

 a vegetative repetition of similar parts ; but should 

 they separate from it in the form of seemingly in- 

 dependent beings, it then becomes difficult to dis- 

 tinguish between the results of fission and gem- 

 mation. Both of these processes are most easily 

 observed among the lower animals. 



Care must be taken not to confound the imme- 

 diate offspring of the true generative act with the 

 detached products of fission and gemmation. The 

 former alone are properly denominated "indivi- 

 duals." The latter may be known by the name of 

 " zooids." Those who apply the term individual to 

 these last are guilty of employing it in two distinct 

 senses, for among the higher animals the apparent 

 individual is always " equal to the total result of 

 the development of a single ovum." ^ 



a 3 



