BINARY FISSION 



63 



surface, it is found that the plane of division is uniform in the different 

 groups. Thus amongst the Mastigophora, which have an anterior flagel- 

 lated end of the body, it is found that in binary fission the body splits 

 longitudinally from before backwards. In those forms in which a cyto- 

 stome is present, as in Chilomastix, in which a dorsal and ventral surface 

 can be distinguished, it is found that a new cytostome is formed near the 

 original one, and if this is also regarded as being on the ventral surface 

 then the body splits longitudinally from before backwards and in a dorso- 

 ventral plane which passes between the cytostomes. In actual division, 

 however, the body often becomes so distorted that it may be difficult to 



Fig. 36. — Successive Stages in Binary Fission of Amceba jtoli/ podia ( x 250). 

 (From Lang, 1901. after Sciiulze, 1875, modified.) 



distinguish the dorsal and ventral surface, though the plane of division 

 may still be regarded as in this direction. In flagellates such as Tricho- 

 monas and the trypanosomes, which possess undulating membranes, 

 division is more complicated (Figs. 160 and 271). A new axoneme grows 

 out from the free half of the divided blepharoplast and passes along the 

 border of the membrane. The membrane then divides between the two 

 axonemes, but the point up to which the membrane has divided at any 

 stage is always a short distance behind the end of the new axoneme. 

 When the new axoneme has reached the end of the membrane the division 

 of the membrane is completed, and the two undulating membranes, each 



