HTDKOZOA. 73 



function to confer individuality upon that which 

 <. previously was but a detached part of the parent 



-r organism. Howsoever complex the body of an 



>%; adult animal may seem, it was once an ovum, 



1 whose extreme simplicity of structure might al- 



>pU e most be said to verge upon homogeneity. What 



sp inaugurated the wonderful series of changes by 



which the ovum fashioned itself into the likeness 

 ^ of its parent? Contact with spermatozoa, or, in 



e 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. How they are capable of 

 exerting this influence is, however, a problem to 

 which, as yet, science has furnished no definite 



tl 



solution. Bischoff 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. 



1 ' The zoological individual being, therefore, de- 



y fined as the entire product of the developmental 



changes of a single fertilised ovum, we have now 

 and :■ to consider the principal modifications which the 

 at cycle of development presents. 



If all the parts of an individual remain mutu- 

 ded. all y connected, its development is said to be 



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



pendent beings, it is ' discontinuous'. 

 , Continuous development may manifest itself 



under the three principal modes of ' growth,' 

 ., « metamorphosis,' and < gemmation without fis- 



1 . sion.' In metamorphosis, growth alternates with 



certain well-marked changes of form. In gem- 

 mation without fission, a tendency to vegetative 



on 



luc 



