484 J. A. DAWSON 



mounts of single animals and of large numbers at one time by 

 smear preparations, sections have been prepared and studied. 



The typical cytoplasmic structure of Oxytricha hymenostoma 

 is alveolar throughout, as is so generally found in hypotrichous 

 forms. A greater or less degree of vacuolization occurs at times, 

 particularly when the vitality is low, as shown by the division 

 rate. The macronucleus consists of two more or less ellipsoidal 

 portions connected by a very delicate strand or commissure. 

 This connection may be seen to best advantage in the living 

 animal, as it almost invariably disappears during fixation. The 

 macronuclei (as the two portions of the macronucleus are fre- 

 quently called) are enclosed in a membrane and consist of chro- 

 matinic and achromatinic elements. The latter do not stain, 

 and if the chromatin is overstained, as frequently occurs, are 

 obscured. Proper differentiation, however, will always show 

 these two elements. A 'Kernspalt' is almost invariably seen, 

 most frequently seeming to bisect each portion of the macro- 

 nucleus, but not uncommonly is found near one end (fig. 8) . A 

 typical specimen of Oxytricha hymenostoma is shown in figure 4. 

 The cytoplasm here is seen to be somewhat vacuolated while 

 the macronuclei have stained deeply. Both nuclei present a 

 somewhat knobby appearance caused by small projecting portions 

 of chromatin. As this condition has occurred frequently, such 

 macronuclei have been most carefully studied in order to make 

 sure that these projections were macronuclear chromatin. The 

 result has been to establish beyond question of doubt that none 

 of these appearances are micronuclei. A diagrammatically typi- 

 cal macronuclear structure is shown in figure 8. 



Throughout the course of the experiments a certain amount 

 of macronuclear fusion has been found, especially in animals 

 in early stages of fission and also at other times. On the other 

 hand, the macronucleus has been observed to be more or less 

 fragmented. This fraginentation has been found to occur more 

 frequently when the division rate was comparatively low. As 

 Woodruff ('05) pointed out, fragmentation of the macronucleus 

 is not necessarily an 'abnormal' condition, since this is a common 

 occurrence during certain stages in the life-history of hypo- 



