



HTDEOZOA. 7 1 



of 



SI 



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 



fie 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 i micropyle.' 

 T 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 



in I highly differentiated portion of the parent or- 

 ganism, the result of a process of development. 



; in: But no sooner has the act of reproduction been 



f j : 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 



^ c ; of the term, an animal individual. 



^ Should the resulting organism develop an 



lit 



F 4 



