REPRODUCTIVE PROCESS IN ANIMALS 61 



organisms this is not so. The living substance that 

 constitutes their body is potentially immortal : provided 

 it gets an appropriate stimulus as a corrective to the 

 period of depression it does not die, but simply divides 

 into two and goes on living as before. 



Now in Nature such a stimulus exists normally in the 

 process of syngamy. If instead of the descendants of 

 one zygote being kept in a vessel by themselves two 

 distinct broods are mixed together there comes a time 

 \vhen the individuals of the two broods become gametes, 

 each fusing with one of the other brood to form a zygote, 

 and each zygote with its new lease of life proceeds to 

 divide over and over again as before. 



There is one other point we should notice before leaving 

 these lowly organized Protozoa. It will be remembered 

 that the zygote of Copromonas normally went through 

 a resting period enclosed in a shell or cyst (Fig. 33 (n) ). 

 The use of this is clearly protective to shield the living 

 zygote from unfavourable external conditions. Now, 

 such a process of encystment is very generally associated 

 with the zygote stage of a Protozoan life-history. And, 

 as we might expect from this, there is often an obvious 

 tendency for the process of syngamy to be associated 

 with some unfavourable change in external conditions 

 such as the drying up of a pool, the absence of food, the 

 coming on of winter. It will be understood that the 

 onset of " unfavourable conditions " may consist either 

 of actual alteration in the external conditions themselves, 

 or of alteration on the part of the living organism itself, 

 so that it gets in some way " out of joint " with its 

 surroundings. 



We may take it as a general principle that the process 

 of syngamy tends to be associated with alteration in the 



