iv REPRODUCTION 283 



testine, the population would soon outgrow the means 

 of subsistence : moreover, when the frog died there 

 would be an end of the parasites. What is wanted in 

 this, as in other internal parasites, is some mode of 

 multiplication which shall serve as a means of dispersal 

 (compare p. 274), or, in other words, enable the progeny 

 of the parasite to find their way into the bodies of other 

 hosts, and so start new colonies instead of remaining 

 to impoverish the mother-country. 



Opalina multiplies by a somewhat peculiar process of 

 binary fission : an animalcule divides* in an oblique 

 direction (Fig. 73, D), and then each half, instead of 

 growing to the size of the parent-cell, divides again 

 transversely (E) . The process is repeated again and again 

 (F), the plane of division being alternately oblique and 

 transverse, until finally small bodies are produced (G), 

 about ^V^sV mm - m length, and containing from two to 

 six nuclei. 



If the parent-cell had divided simultaneously into a 

 number of these little bodies the process would have 

 been one of multiple fission (p. 254) : as it is, it forms an 

 interesting link between simple and multiple fission. 



Opalina multiplies in this way especially rapidly in 

 the spring i.e., during the frog's breeding season. 

 The majority of the small products of division (G) be- 

 come encysted (H), and in this passive condition are 

 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 frog's 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 divide up into lanceolate 

 gametes each containing a single nucleus and covered 



