PHENOMENA ACCOMPANYING FERTILIZATION 303 



Other exceptions are found in Euplotes patella in all Vorticellidae 

 and Ophryoscolecidae examined up to the present time. Here the 

 micronucleus undergoes one or more preliminary mitoses prior to 

 the first meiotic division. 



In eiliates with two micronuclei both undergo the first maturation 

 division. According to Prowazek (1899) the 4 resulting nuclei of 

 Stylonychia pustulata divide again, thus forming 8 products at the 

 second division. According to Maupas (1889), however, 2 of the 

 first 4 nuclei of Stylonychia pustulata, and of Onychodromus grandis 

 as well, degenerate so that only 2 second maturation nuclei are 

 formed. Gregory's (1923) observations indicate that a variable 

 number take part in the second division of Oxytricha fallax. 



In forms with many micronuclei in the vegetative stage there 

 seems to be no general rule as to the number which undergo a 

 second division. Prandtl found a variable number in Didinium 

 nasutum, Prowazek a large number in Bursaria truncatella, and 

 Calkins a variable number in Uroleptus mobilis; while 1 and 4 

 nuclei are rarely found, 2 or 3 are characteristic. 



In summing up the accumulating evidence on meiotic phenomena 

 in the eiliates the conclusion may be drawn that the history in the 

 main is similar to the history of meiosis in Metazoa. Chromo- 

 somes of definite number are characteristic of most species and this 

 number is reduced to one-half during one or the other of the two 

 divisions. 



4. Phase!). The Third Division. Pronuclei Formation. — A third 

 division of the nuclei subsequent to reduction in number of chromo- 

 somes is characteristic of all eiliates 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. 319). In 

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

 if judged by Metazoa, 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. 153). 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 two functional pronuclei. Anoplophrya branchiarum, Para- 

 mecium caudatum, Chilodon uncinatus, Colpidium colpoda, Leuco- 

 phrys patula, Glaucoma scintillans, Loxophyllum meleagris, Spiro- 

 stomum teres, Bursaria truncatella, Blepharisma undulans, Boveria 



