, PHENOMENA ACCOMPANYING FERTILIZATION 527 



4. Phase D. The Third Dnision. Pronvcki Formation.— A 

 third division of the nuclei subsequent to reduction in number of 

 chromosomes is characteristic of all ciliates in which fertilization 

 has been carefully studied. It is extremely difficult to interpret 

 this final division which gives rise to the pronuclei (see infra p. 542). 

 In the majority of cases it appears to be a transverse division which, 

 if judged by INIetazoa, would make it a second reduction division. 

 One of the products is a wandering pronucleus which migrates, 

 the other is a stationary pronucleus which ultimately fuses with the 

 migratory pronucleus from the other individual. There is some 

 evidence that the migrating pronucleus is equivalent to a spermato- 

 zoon (Dogiel, 1925). 



The third division spindles are always characteristic and different 

 from the spindles of the meiotic divisions. Not only are they fre- 

 quently heteropolar, but the late telophase state is characterized 

 by long connecting strands of nuclear substance (Fig. 31, p. 71). 

 There is no uniformity in regard to the niunber of nuclei to undergo 

 this third division although only one of the dividing nuclei provides 

 the two functional pronuclei. Anophphrya hranchiarum, Para- 

 mecium caudatum, Chilodon uncinatus, Colpidiwm colpoda, Leiico- 

 phrys patvla, Glaucoma scintillans, Loxophylhnu vieJeagris, Sjnro- 

 stomiim feres, Bvrsaria tmncateJla, BJepharisma vndidans, Boreria 

 suhcyJindrica, Liomiius fasciola, and in the VorticeJlidce, only 1 

 nucleus undergoes this third division. In Onychodromiis grcmdis 

 Stylonychia pnstidata, and Euplotes patella, 2 nuclei, in Oxytricha 

 falla.v (Gregory) 2 or 3, and in Uroleptns mohilis, 2, 3 or 4 nuclei, 

 undergo the third division. 



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

 the w^andering and the stationary pronuclei ( Didinium nasidinu), 

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

 of Paramecium caudatvm and were able to trace this difference back 

 to a heteropolar third flivision 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 ad\'ancing pronucleus of Euplotes patella and 

 Prandtl, more pronounced astral radiations about the wandering 

 pronucleus of Didinium nasutum. In Uroleptvs mohilis such 

 radiations are absent but a fairly homogeneous condensed "sphere" 

 of cytoplasmic substance precedes the wandering pronucleus in its 

 migration. (Fig. 219). 



What is the significance of this third division? The answer can 

 be only speculative at the present time. The absence of definite 

 chromosomes in some cases, e. g., Paramecium, and the occurrence 

 of heteropolar mitotic figures lend some support to the view that it 

 is a differential division whereby male chromatin, as suggested 



