HYDROZOA. 7 1 



from their bearing on the subject of animal deve- 

 lopment in general. A few words of explanation 

 may therefore, in this place, not appear unneces- 

 sary. 



The life of every animal species may, from a 

 certain point of view, be regarded as consisting in 

 the alternate performance of two distinct series 

 of acts; the one of reproduction, the other of 

 development. 



Each act of reproduction consists essentially in 

 this, that two dissimilar bodies, an ovum and a 

 spermatozoon, are brought into mutual contact. In 

 some cases the spermatozoon penetrates the coats 

 of the ovum, or even enters it by a proper aper- 

 ture, known as the * micropyle.' 



Thus defined, the process of reproduction is the 

 same in all animals, though in some its simplicity 

 is masked by the occurrence of a variety of other 

 phenomena, all, however, of secondary importance. 



It must also be borne in mind that the evolu- 

 tion of ova and spermatozoa, obviously necessary 

 as a prelude to the reproductive function, cannot 

 be considered as forming a part of it. An ovum 

 or spermatozoon is, in truth, nothing more than a 

 highly differentiated portion of the parent or- 

 ganism, the result of a process of development. 



But no sooner has the act of reproduction been 

 duly effected, than that of development forthwith 

 begins. The fertilised ovum gives rise to an 

 embryo, which tends to evolve itself into the like- 

 ness of its parent. This embryo, together with 

 all the structures subsequently developed there- 

 from, is said to constitute, in the zoological sense 

 of the term, an animal individual. 



Should the resulting organism develop an 



F 4 



