INTRODUCTORY : THE ANIMAL ORGANISM 9 



of an animal is a cycle, in which it passes through a series 

 of stages, beginning with the small and simple reproductive 

 body, and ending with the larger and usually more com- 

 plex adult, ready to undergo fission again. Every individual 

 goes through the same cycle of changes as its parent, re- 

 sembling in each stage a similar stage passed through by 

 the latter, till it reaches the likeness of the individual 

 that produced it. This is due to the property known as 

 heredity. In a later chapter we shall have occasion to 

 examine heredity more closely. 



It will be seen that, in the strict sense of the word, re- 

 production includes the whole life-cycle, and 

 Reproduction, consists of two distinct processes — fission, and 

 the development of the reproductive body into 

 the adult — for until this cycle has been completed the 

 parent is not re -produced. 1 



The meaning of growth will now be apparent. It is that 

 part of the process of development by which the repro- 

 ductive body reaches the size of the adult. At the same 

 time, in most cases, and perhaps in all, the growing 

 individual is undergoing 



the changes in structure ^-—^* Adult 



to which we have al- 

 luded. 



Here must be men- 

 tioned a pro- 

 Conjugation. / . , 

 cess which 



it will not be possible ^Reproductive Body • 



properly to discuss till Fig. *.— A diagram of the life-cycle 



some examples of it have of an animal, 



been studied in detail. It 



is well known that in most animals reproduction is only pos- 

 sible by the co-operation of two individuals of different kinds, 

 known as the sexes. This is because in such animals the 

 reproductive bodies are of two sorts, each produced only 

 by one of the sexes, and neither sort can develop except 

 after fusion with one of the other sort. That fusion is an 

 example of the process known as conjugation. From time 

 to time there occurs in nearly all animals such a union of 



1 Development may partly take place before fission, as m many 

 cases of budding (Chap. XI.). 



