632 



FERTILIZATION 



with multiple micronuclei such as Uroleptus mobilis (Calkins, 1919), a 

 variable number of nuclei may divide a second time. In Euplotes patella 

 all micronuclear products undergo a second meiotic division. Because 

 of the preliminary division, there are four in each conjugant. No resting 

 stage occurs between divisions here, in contrast to MacDougall's (1936) 

 account of Chilodonella. The daughter nuclei of the previous division 

 are still connected by their respective drawn-out nuclear membranes when 

 the chromatin begins to resolve itself into a reticulum in each nucleus 

 and the granules on the reticulum condense into eight discrete, ovoid 



Figure 150. Second meiotic (reduction) division in Euplotes patella. A, eight ovoid 

 chromosomes appearing on the spindle; B, synaptic pairing and lengthening of chromo- 

 somes; C, disjunction and separation of homologous chromosomes in the anaphase, four 

 passing to each pole. 



chromosomes (Fig. 150 A). The chromosomes now conjugate in four 

 pairs, in what is probably a delayed synapsis, elongate somewhat, and 

 disjoin longitudinally, four haploid chromosomes passing to each pole. 

 Calkins (1919) described a similar pairing and separation of the 

 eight chromosomes in Uroleptus mobilis. Tannreuther (1926) presents 

 evidence of chromosome pairing in the reduction division of Prorodon 

 griseus, but in most cases where synapsis has been observed, it occurs in 

 the first meiotic division. 



STAGE D, THE THIRD DIVISION, AND THE FORMATION OF PRONUCLEI 



In all ciliates thus far studied, a third division occurs. This division 

 is equational in character and usually involves only one nucleus, while 

 the rest degenerate. The two products of this division are the pronuclei 

 which take part in fertilization. 



In a few ciliates, two, three, and four micronuclei have been reported 

 to divide at this stage, but in no case has it been demonstrated that the 



