THE NUCLEI OF UROLEPTUS MOBILIS 



347 



leptis, etc.)- There is no uniformity in regard to the number of 

 nuclei to undergo this third division, although only one of the 

 dividing nuclei provides the functional pronuclei. In Anoplo- 

 phrya, Paramecium, Chilodon, Colpidium, Leucophrys, Glau- 

 coma, Loxophyllum, Spirostomum, Bursaria, Blepharisma, Bo- 

 veria, Lionotus, and in the vorticellidae only one spindle is 

 formed for this third division. In Onychodromus, Stylonychia, 

 and Euplotes, according to Maupas ('88), two spindles are pres- 

 ent and four pronuclei are formed, two of which degenerate and 

 disappear (in Stylonychia, according to Prowazek ('99), only one 

 third division spindle is present) . Uroleptus mobilis differs from 

 the majority of other ciliates in having from two to four nuclei 

 undergoing this third division at the same time, and as many as 

 eight products of this division may be present in the cell before 

 any two are metamorphosed into pronuclei (fig. Ql). 



e. The pronuclei. Prandtl ('06) was the first to note a differ- 

 ence in size between the wandering and the stationary pronuclei 

 of Didinium nasutum. Calkins and Cull ('07) noted a similar 

 difference in pronuclei of Paramecium caudatum, and were able 

 to trace this difTerence back to a heteropolar third-division spin- 

 dle. Other observers have failed to find any constant differ- 

 ences, and this is the case in Uroleptus mobilis, where the size 

 relations of the fully formed pronuclei, as carefully measured in 

 ten individuals, are as follows: 



* Pairs 1 to 4 were fixed in Bouin's fluid; pair 5 in sublimate acetic. 



