The Life History of Tillina Magna 395 



ually lost, the peristomial region disappears, and the posterior 

 lobe is absorbed and the large elongated macro-nucleus is shifted 

 in position. If, in the changing of position, one end of the nucleus 

 is moved, the U-shaped form, already mentioned, will result. 

 The spherical nucleus may also be explained as a result of the 

 rotation. In these cases, however, the nuclear changes have 

 anticipated division, as the membrane is not yet formed. The 

 mouth, as well as the nucleus, is changed in its position. Finally, 

 when the spherical form has been reached, nothing is visible 

 save the nucleus, contractile vacuole, food vacuoles and the 

 cortical layer, which is still prominent just below the membrane. 

 Biitschli states that the presence of this layer within the cyst 

 is the exception rather than the rule. There is some question 

 in regard to the history of the mouth during the formation of divi- 

 sion cysts. The mouth is not visible in the living cysts, possibly 

 being concealed by the presence of many food vacuoles; neither is 

 it always found in sections of the division cysts. It seems most 

 probable that the old mouth disappears at an early stage, and that 

 a new one is formed before the appearance of the first plane of 

 division. This would explain the fact that in sectioned material 

 a mouth is found in some sections and not in others. This division 

 cyst differs from that of Colpoda cucullus in being always spherical 

 in form, and in having no definite opening in the cyst wall. 



The first indication of division is in the elongation of the nucleus 

 w4th a slight constriction in the center (Plate II, Fig. l). Almost 

 at the same time a constriction appears in the membrane, extend- 

 ing toward the center in such a way that the division plane will 

 pass through the nucleus and the mouth. The contractile vacuole 

 lies at one side, and passes to one of the daughter halves, a new 

 one being formed in the other half. After the first division, or 

 more often, before the first division has been completed, there is a 

 shifting of the daughter nuclei and mouths, and the second plane 

 of division appears at right angles to the first (Plate II, Figs. 2, 3,4). 

 During this process the macro-nucleus divides by simple division. 

 In some cases the chromatin stands out in sharp contrast to the 

 light faintly staining achromatic ground substance. The chro- 

 matin maybe arranged in lopes or masses, in the central portion of 



