THE ANIMAL ORGANISM 



they never resemble them in every detail. The succession of 

 changes which brings this about is called development, and is 

 sometimes straightforward, or direct, sometimes, as in the well- 

 known case of the butterfly, very roundabout, or indirect. In 

 reproduction by budding (Chap. 9) development may take 

 place partly or mainly before fission. Thus the life of an animal 

 or plant 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 complex adult, ready 

 to undergo fission again. Every individual goes through the same 

 cycle of changes as its parent, resembling in each stage a similar 

 stage passed through by the latter, till it reaches the likeness 

 of the individual that produced it, that is, it shows the property 

 known as heredity. Thus, in the strict sense of the word, repro- 

 duction includes the whole life cycle and 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 reproduced. From this point of view, growth is 

 that part of the process of development by which the reproductive 



body reaches the size of the adult. At the 

 same time, usually, and perhaps always, the 

 growing individual is undergoing the changes 

 in structure to which we have alluded. 



SYNGAMY 



Here must be mentioned a process which 

 is an essential part of reproduction in many 

 organisms and in all the higher animals. In 

 such organisms 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 a syngamy, union of two 

 distinct living bodies, which occurs from 

 time to time in nearly all species of animals 

 and plants. The bodies which unite are 

 known as gametes, and that which results 

 from their fusion as a zygote. In some of 

 the smallest living beings (Fig. 2) syngamy 



Fig. 2. — Copromonas, a 

 minute inhabitant of 

 dung. — After Dobell. 



a, Adult individual ; b, the same 

 in fission ; c, two adult in- 

 dividuals in syngamy; 

 d, the zygote, enclosed in a 

 cyst. 

 .v.. Food vacuole;^?., flagellum ; 

 g., gullet; nu., nucleus; res., 

 reservoir of contractile 

 vacuole (see p. 41), 



