SPIROSTOMUM AMBIGUUM 421 



with the anterior end of the meganucleus. This is the beginning 

 of the daughter contractile vacuole. 



A gradual elongation of the meganucleus next takes place 

 (Text-fig. 8). A curious phenomenon in the elongation of the 

 meganucleus, observable also in its contraction, is that this 

 process does not take place equally fast at each end of the 

 mass. Elongation takes place more rapidly in the posterior 

 part than in the anterior, so that while the posterior half of the 

 animal has quite a long developing meganucleus only a short 

 part projects into the anterior half. The elongating mega- 

 nucleus does not expand in a straight line but is often coiled in 

 its course. Both anterior and posterior ends have the shape 

 of a crook, which often persists even after the separation of the 

 two daughters. 



During the elongation of the meganucleus a slight con- 

 striction of the body can be observed a little posterior to the 

 anterior cytostome. This gradually becomes more pronounced 

 and marks the point of the future separation of the two 

 daughters. 



Coincident with the development of this constriction is 

 the gradual enlargement of the dilation in the feeding canal 

 of the contractile vacuole. For some time it continues to 

 empty wdth the contents of the original vacuole, but a con- 

 siderable length of time before the separation of the two 

 daughters it becomes disconnected from the posterior part of 

 the canal and contracts independently of the posterior vacuole. 

 This separation of the anterior contractile vacuole from the 

 posterior one seems to take place as soon as the constriction 

 of the cytoplasm is sufficiently deep to allow the excretion 

 of the fluid through a pore in the median line of the con- 

 striction. 



Although the meganucleus in the anterior half seems to 

 grow more rapidly immediately prior to the separation of the 

 two daughters than that in the posterior part, the length cf 

 meganucleus in each daughter at the time of separation is 

 still a trifle unequal (Text-fig. 9). This inequality seems to 

 be adjusted later. The meganucleus divides into the two 



NO. 267 F f 



