CONJUGATION IN BLEPHARISMA UNDULANS 679 



for, and it seems hardly probable that they, or the spindle fig- 

 ures giving rise to them, should have been overlooked. On 

 the other hand they are so minute and so difficult to stain that 

 this possibility must remain open. 



The first evidence that I have found as to maturation divi- 

 sions is the late stage in mitosis of the four functional micro- 

 nuclei (fig. 12). They lie close together in the anterior third 

 of the cell, the daughter nuclei appearing first as homogeneous 

 chromatin then as vesiculate nuclei. One of the eight result- 

 irg nuclei divides again, forming the two pronuclei, while the 

 other seven swell, degenerate and finally disappear (fig. 14). 

 The two pronuclei are practically the same in size and appear- 

 ance, although in one instance the two migratory nuclei were 

 larger than the stationary (fig. 13). At the time of fusion the 

 pronuclei are elongate and spindle formed, union taking place 

 first at the ends (figs. 14 and 15). After fusion the syncaryon 

 becomes spherical and the chromatin is uniformlv distributed 

 (fig. 16). 



The interchange and fusion take place at the posterior limits 

 of the zone of coalescence and the fertilization nuclei remain in 

 this region during their further changes. Each nucleus forms 

 a spindle and divides by typical mitosis (figs. 18 and 19), the 

 full spindle figure (fig. 19) being much larger than any of 

 the maturation nuclei and showing typical fibers and chro- 

 matin. The two nuclei in each cell resulting from this first 

 division do not pass immediately into the second spindle stage 

 but evidently remain for some time in the resting stage, since 

 they are often found in various positions relative to the old 

 macronucleus (figs. 21, 22 and 23). Each nucleus then divides 

 again by mitosis, this being the stage described by Biitschli. 



The period of nuclear quiescence after the first division is 

 characterized, in the majority of cases, by the commencement 

 and completion of separation of the conjugating cells. This 

 is accomplished by the progressive vacuolization of the proto- 

 plasm in the fine of fusion. The animals separate, first in the 

 center of the fused areas by coalescence of the vacuoles, then at 

 the posterior or oral region, and finally at the anterior tips. 



