LIFE HISTORY OF PRORODON GRISEUS. 305 



some substance which helps to dissolve the cyst wall, as it 

 becomes thinner before their escape. No experiments were made 

 to determine how long the encysted animals were able to survive 

 under adverse conditions. A very small part of their life history, 

 however, is passed in the free-living condition, even under 

 favorable conditions as verified in the laboratory. 



MATERIAL AND METHODS. 



The material was fixed at various stages in the Life history of 

 the animal. Bouin's fluid was found to give satisfactory results. 

 The animals were removed from the cultures with the aid of a 

 pipette avoiding excess water, and spurted into the tubes with the 

 killing fluid. The centrifuge was quite helpful in concentrating 

 the animals at one end of the tube. The specimens were stained 

 in Delafield's hematoxylin or Tulodin blue. Either stain gave 

 good results. Erythrosin was added to the ninety-five per cent, 

 alcohol for cytoplasmic differentiation. The animals were then 

 dehydrated, cleared in xylol and mounted in balsam. For 

 sections the animals were imbedded in paraffine, sectioned and 

 stained on the slide. The sections were quite helpful in checking 

 the results as found in the whole mounts. 



BINARY FISSION. 



Reproduction by binary fission is quite common. It may occur 

 during encystment (Fig. 2d), but it is found more abundant 

 during the free active stages. Transverse division in the encysted 

 stage is rather difficult to verify in the whole mounts, since the 

 division of the cytoplasm as a rule can not be recognized except 

 in sections. Division in the free-swimming stage is more distinct 

 and the different stages can be followed step by step. The 

 animal shortens somewhat on its long axis. The nuclear 

 activities (Figs. 4-6) are similar to those of the encysted forms. 

 The micro-nucleus as is common in ciliates divides first before 

 division of the macro-nucleus. The macro-nucleus varies con- 

 siderable in shape from a spherical to a somewhat elongated or 

 curved condition. The elongated nucleus in binary fission often 

 becomes constricted by a nuclear cleft without any further 

 elongation and half going to either new daughter cell (Fig. 6). 



