CONJUGATION IN BLEPHARISMA UNDULANS 673 



preparations of the dividing cell it is possible to get them in 

 profile, when they appear associated with the macronucleus 

 very much as do the micronuclei of ordinary ciUates like Para- 

 mecium or Colpidium. The relations come out best in sec- 

 tions and especially sections of late stages in division. In 

 early stages they are apparently within the nuclear membrane 

 and are difficult to distinguish from the thick, granular chro- 

 matin of the macronucleus. Here and there, however, in the 

 mass of chromatin, larger and more definite deeply staining 

 bodies may be seen which may be the micronuclei (fig. 3). In 

 later stages they become much more definite, although still 

 small and inconspicuous. They lie close against the chroma- 

 tin of the macronucleus and within the delicate nuclear mem- 

 brane surrounding this organ. In some cases they are separated 

 b}^ a slight space, but this condition is exceptional. Ordinarily 

 they lie in pairs (fig. 8), the highest number that I have seen 

 being eight, or four pairs. The remarkable significance of this 

 intranuclear position comes out in connection with the phe- 

 nomena of conjugation to be described later. 



The macronucleus, usually so definite in form and simple in 

 its changes during vegetative life and division, is extremely 

 difficult to interpret in the case of Blepharisma undulans. Stein 

 described B. undulans as having two macronuclei, one in each 

 half of the cell and with or without connecting chromatin threads. 

 This condition is fairly common, but so also is the uninucleate 

 condition and the multinucleate condition, while interme- 

 diate phases connect these two extremes. The single macro- 

 nucleus is compact and homogeneous and may or may not 

 have the characteristic 'Kernspalt' common to many of the 

 ciliates. A common occurrence is the presence of two or three 

 macronuclei grouped together as shown in figure 1, and in these 

 cases there is a difference in texture between the different ele- 

 ments. I am inclined to interpret these as incomplete stages in 

 the fusion of the portions of the macronucleus after division, for 

 the nucleus during division always begins as a unit, stretching 

 out as a single strand and breaking up into numerous portions 

 at the end of division (figs. 2, 7, 8). The difference in texture 



