PHENOMENA ACCOMPANYING FERTILIZATION 



i05 



subcylindrica, Lionotus fasciola, and in the Vorticellidae, only 1 

 nucleus undergoes this third division. In Onychodromus grandis, 

 Stylonychia pustulata, and Euplotes patella, 2 nuclei; in Oxytricha 

 fa I lax (Gregory), 2 or 3, and in Uroleptus mobilis, 2, 3 or 4 nuclei, 

 undergo the third division. 



Prandtl (190(5) was the first to note a difference in size between 

 the wandering and the stationary pronuclei (Didinium nasutum), 

 Calkins and Cull (1907) described a similar difference in pronuclei 

 of Paramecium caudatum and were able to trace this difference back 



Fig. 



154.— Uroleptus mobilis, conjugation. The interchange of pronuclei, each 

 preceded by a characteristic "attraction sphere." (After Calkins.) 



to a heteropolar third division spindle. In other cases there seems 

 to be no characteristic difference in size between the two pronuclei 

 although other differences may be evident. Thus Maupas noted 

 the presence of a dense aggregate of cytoplasmic granules at the 

 forward pole of the advancing pronucleus of Euplotes patella and 

 Prandtl, more pronounced astral radiations about the wandering 

 pronucleus of Didinium nasutum. In Uroleptus mobilis such radi- 

 ations are absent, but a fairly homogeneous condensed "sphere" 

 of cytoplasmic substance precedes the wandering pronucleus in its 

 migration (Fig. 154). 

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