HYDIIOZOA. 73 



fimction to confer individuality upon that which 

 previously was but a detached part of the parent 

 organism. Howsoever complex the body of an 

 adult animal may seem, it was once an ovum, 

 whose extreme simplicity of structure might al- 

 most be said to verge upon homogeneity. What 

 inaugurated the wonderful series of changes by 

 which the ovum fashioned itself into the likeness 

 of its parent ? Contact with spermatozoa, or, in 

 one word, reproduction. To say, then, that sper- 

 matozoa possess a peculiar individualising in- 

 fluence can scarcely be viewed as a metaphorical 

 form of expression. Hoio they are capable of 

 exerting this influence is, however, a problem to 

 which, as yet, science has furnished no definite 

 solution. Bischofif has compared their action to 

 that of a ferment, such as the yeast of beer ; but 

 this hypothesis, as Claparede truly observes, only 

 removes the present difficulty a single step back- 

 wards. 



The zoological individual being, therefore, de- 

 fined as the entire product of the developmental 

 changes of a single fertilised ovum, we have now 

 to consider the principal modifications which the 

 cycle of development presents. 



If all the parts of an individual remain mutu- 

 ally connected, its development is said to be 

 ' continuous ' ; if any of them separate as inde- 

 pendent beings, it is ^discontinuous'. 



Continuous development may manifest itself 

 imder the three principal modes of * growth,' 

 'metamorphosis,' and 'gemmation without fis- 

 sion.' In metamorphosis, growth alternates with 

 certain well-marked changes of form. In gem- 

 mation without fission, a tendency to vegetative 



