THE FLAGELLATA 199 



During the asexual period the monads feed greedily and 

 grow to a large size before division. An individual about to 

 enter on division becomes sluggish in its movements, and its 

 flagellum, waving about languidly the while, is gradually 

 withdrawn into the body until it disappears altogether. While 

 the flagellum is being withdrawn the cytostome and cyto- 

 pharynx also become indistinct, and appear to close up and 

 become obliterated. Meanwhile the nucleus has been under- 

 going changes. It elongates in a direction transverse to the 

 long axis of the body, and first becomes fusiform and subse- 

 quently dumbbell-shaped (fig. 42, >\ the latter shape being 

 due to the massing of the chromatin particles at the two ends 

 of an elongated achromatic band. In a short time two new 

 flagella make their appearance at the anterior end. At first 

 they are minute peglike outgrowths, but they soon increase 

 in length and as they do they begin to execute the characteristic 

 lashing movements, as a result of which the erstwhile indolent 

 dividing monad again becomes active. A cleft now makes 

 its appearance at the anterior end between the bases of the 

 sprouting flagella : first it cuts the reservoir in two and thence 

 extends gradually backwards till it divides the animalcule 

 longitudinally into halves. While these changes are in 

 progress the dumbbell-shaped nucleus of the earlier stage 

 assumes the shape shown in fig. 42, C, and finally the thread 

 connecting the two chromatin masses breaks and the latter 

 become rounded off to form the nuclei of the two daughter- 

 monads. The nuclear division is therefore amitotic. The 

 cytostome and cytopharynx appear to be formed anew 

 in each product of division. It is evident that a 

 moiety of the reservoir of the parent form goes to each 

 daughter form, but the contractile vacuole of the parent 

 persists and passes over to one of the daughter forms, the 

 other developing a new contractile vacuole. 



It is not clear what is the limit, or what imposes a limit, on 

 the capacity of a swarm of Copromonas to reproduce asexually 

 by binary division. It may be due to an exhaustion of the 

 food supply, or to a chemical change in the medium in which 

 they are living, or to an inherent incapacity to reproduce 

 themselves asexually for more than a certain number of 

 generations ; but whatever the cause, after a period varying 

 from two to six days, the monads begin to conjugate. In 



