XI DEVELOPMENT 125 



iDer of these little bodies the process would have been one of 

 multiple fission : as it is, it forms an interesting link between 

 simple and multiple fission. 



Opalina ranarum multiplies in this way in the spring— />. 

 during the frog's breeding season. Each of the small pro- 

 ducts of division (g) becomes encysted (h), and in this 

 passive condition is passed out with the frog's excrement, 

 probably falling on to a water-weed or other aquatic object. 

 Nothing further takes place unless the cyst is swallowed by 

 a tadpole, as must frequently happen when these creatures, 

 produced in immense numbers from the frogs' eggs, browse 

 upon the water-weeds which form their chief food. 



Taken into the tadpole's intestine, the cyst is burst or 

 dissolved, and its contents emerge as a lanceolate mass of 

 protoplasm (i), containing a single nucleus and covered with 

 cilia. This, as it absorbs the digested food in the intestine 

 of its host, grows, and at the same time its nucleus divides 

 repeatedly (k) in the way already described, until by the time 

 the animalcule has attained the maximum size it has also 

 acquired the large number of nuclei characteristic of the 

 genus. 



Here, then, we have another interesting case of develop- 

 ment (see p. 43) : the organism begins life as a very small 

 uninucleate mass of protoplasm, and, as it increases in size, 

 increases also in complexity by the repeated binary fission 

 of its nucleus. 



