SPIROSTOMUM AMBIGUUM 417 



in the small variety and one in the large variety. As he relied 

 upon freshly collected material this was not strange, for even 

 in healthy cultures individuals undergoing division often are 

 not numerous. This is, no doubt, due to the fact that a period 

 of two to three days elapses between divisions even in an 

 animal subjected, as far as it is possible for us to tell, to excellent 

 conditions. Divisions takes place during the night as well as 

 in the day time ; for in the morning usually there are present 

 in the cultures a number of animals in the last stages of division, 

 or some which have recently divided. 



As the process of division differs very little in the two varieties, 

 it is unnecessary to give a description of each ; it will be suffi- 

 cient to describe it as it occurs in the major variety and to note 

 any deviation which occurs in the minor variety. 



In order to follow the entire process of division it was found 

 best to isolate individuals which showed the first signs of 

 coming fission and to observe them through the whole process. 

 Stained preparations were made at different stages. 



The time taken to complete the phenomenon varies in 

 different individuals, but the average time is from seven to 

 eight and a half hours ; even then, the two daughters, though 

 separated, have not attained the normal form. This is 

 especially true of the form of the meganucleus. Simpson {21) 

 gives one to two hours as the time required for division in 

 Spirostomum ambiguum; but this I take to mean the 

 actual division of the animal's body into two parts irrespective 

 of nuclear changes. 



In the major variety the mouth, in animals about to undergo 

 division, lies midway between the anterior and posterior ends. 

 Large numbers of animals belonging to the major variety have 

 been observed undergoing division, and in every case the 

 mouth of the parent was at the middle of the body length. 

 In his single observation on fission in this variety Stein describes 

 the old peristome as extending through the anterior two-thirds 

 of the body length, and the new peristome of the future daughter 

 as developing in the posterior third of the body. He further 

 states that after half an hour's observation the beginning of 



