424 ANN BISHOP 



lias been in the manner described above. When they are 

 attached in the manner described by Stein (28) the posterior 

 ends are level, since the conjugants are almost without exception 

 equal in size ; when they are attached in the more common 

 manner, however, the posterior end of one individual projects 

 beyond that of the other. Observations of individuals just 

 beginning to conjugate showed that the anterior ends were the 

 first to become attached. Sections show that the conjugants 

 are joined by a thin sheet of ectoplasm and that the endoplasm 

 of the individuals does not mingle. 



The contractile vacuole seems to function in a normal 

 manner during conjugation, though the average time of its 

 contraction was not studied. 



One of the difficulties encountered in studying conjugating 

 pairs is that newly attached individuals often become separated 

 in drawing them up a pipette. Such severed conjugants have 

 never been observed to become reattached but sooner or later 

 die. Maupas (19) experienced similar difficulties whilst working 

 upon conjugation in S. teres. Whilst working upon Para- 

 moecium, however, Calkins found that conjugants, if severed 

 before there had been any exchange of nuclear material, 

 would live and divide in a normal manner. Similar experiments 

 upon artificially severed conjugants were carried out by 

 Baitsell (1), but without exception the severed conjugants all 

 degenerated and died within the twenty-four hours following 

 the operation. 



The time taken from the attachment of a pair of conjugants to 

 the time of their separation varies between sixty and seventy-two 

 hours. It is very difficult to be absolutely certain of the dura- 

 tion of conjugation in a pair, since it is practically impossible 

 to remove for observation a pair which are only just becoming 

 attached. They invariably become separated during the 

 removal from the culture to the depression slide. 



In order to secure permanent preparations of as many stages 

 of conjugation as possible, the conjugants were removed from 

 the culture and placed into test-tubes, which contained some 

 of the culture solution, and fixed at different intervals. This 



