REPRODUCTIVE PROCESS IN ANIMALS 57 



new individuals under favourable conditions behaves 

 just like its parent : it feeds, grows and eventually divides 

 again by fission. And so generation after generation- 

 each individual ends by resolving itself into two new 

 individuals. 



But this process of fission does not go on indefinitely. 

 From time to time it is interrupted by an opposite kind 

 of process in which, in place of one individual becoming 

 divided into two, two individuals become fused into a 

 single one in the process known as Syngamy illustrated 

 by the lower part of Fig. 33. Two individuals swimming 

 about come in contact by their front ends (6), adhere 

 together and become gradually merged into a single 

 individual shaped like the parent (7-10). An essential 

 feature of this process of syngamy is that the nuclei of 

 the tw^o individuals, after undergoing complicated changes 

 which need not be gone into, undergo complete fusion 

 together (10, u), just as the protoplasmic bodies do. 



In the process just described we recognize a typical 

 case of syngamy : the t\vo individuals which fuse together 

 are gametes ; the single individual produced by their 

 fusion is a zygote. A very usual sequel to the process 

 of syngamy is well seen in Copromonas, in that the zygote 

 rounds itself off, surrounds itself with a protective shell 

 or cyst (n), and enters on a period of repose before it 

 emerges again and resumes its pear-shape and its active 

 swimming existence. 



One of the most interesting things about Copromonas 

 is that while it shows typical Syngamy the essential 

 phenomenon of Sex there is no obvious sexual difference, 

 no obvious sign of maleness or femaleness. To study 

 these differences we will take two other minute Protozoa- 

 (i) Stylorhynckus, which lives as a parasite in the intestine 



