PHENOMENA ACCOMPANYING FERTILIZATION 313 



whether or not new cirri, membranelles, and undulating membranes 

 are formed and the old ones absorbed, has not been fully determined 

 by observation but this appears to be the case in Uroleptus mobilis. 

 The most important of the changes at this period have to do with 

 the formation of the new macro- and micronuclei. The inaccurate 

 statement is often made to the effect that the new macronucleus 

 is formed by the metamorphosis of a micronucleus. This is strictly 

 true only in cases of parthenogenesis. In fertilization both macro- 

 and micronucleus are formed from products of the amphinucleus, 



Fig. 159. — Uroleptus mobilis; conjugation at the stage of nuclear fusion: g, n, 

 gametic nuclei about to fuse; B, same enlarged; C, elongation of amphinucleus 

 shortly after fusion. (After Calkins.) 



and both types of nuclei are formed by metamorphosis of such 

 products. In the majority of cases the first metagamic division 

 of the amphinucleus results in two equivalent nuclei. In Uroleptus 

 mobilis this division occurs very soon after fusion and before com- 

 plete mixture of the two pronuclei is established (Fig. 159). This 

 is shown by the occasional finding of nuclei in which 4 of the 8 

 chromosomes are in the anaphase stage while the other 4 are in the 

 metaphase (Fig. 160). The two products of this division have 

 different fates. One of them divides again to form two nuclei 

 which lose their vesicular character and condense into minute and 



