CONJUGATION IN VORTICELLA 241 



single micronucleus of fusiform shape, measuring some 4-5 /x 

 in its longer and 3 //. in its shorter diameter. Scarcely is it 

 fixed, however, to a macrogamete, and before actual fusion has 

 taken place, than the micronucleus undergoes mitosis and 

 divides into two. The micronucleus of the macrogamete mean- 

 while remains inert. In the next stage the micronuclei are un- 

 altered, but the microgamete loses its posterior circlet of cilia 

 and becomes intimately fused with the macrogamete. When 

 this is accomplished the micronuclei in both gametes increase 

 greatly in size, become spindle-shaped, and divide by mitosis. 

 A second division follows, and as there were two micronuclei 

 in a microgamete and one in the macrogamete the result is 

 that there are eight micronuclear corpuscles in the former and 

 four in the latter. Up to this time the micronucleus of the 

 macrogamete and its products have been situated in the 

 anterior region of the body. The peristome being closed, 

 water now accumulates below it and forces the four micro- 

 nuclear products towards the posterior end, so that they are 

 closely contiguous to the micronuclear corpuscles of the 

 microgamete. Seven of the last-named then disintegrate 

 and disappear, the one remaining moving close up against 

 the partition which still exists between the bodies of the 

 macrogamete and microgamete. Similarly three of the four 

 micronuclear corpuscles of the macrogamete disappear, and 

 the one remaining takes up a position opposite to the surviv- 

 ing corpuscle in the microgamete. These corpuscles increase 

 notably in size, the dividing wall separating them disappears ; 

 they come into contact, and, on doing so, both divide mitotic- 

 ally. The result of the division is that one product of each 

 micronucleus is pushed back into the body of the microgamete, 

 there to disintegrate and be absorbed without leaving a trace. 

 The two remaining products, constituting the male and 

 female pronuclei, are pushed into the body of the macrogamete, 

 where they fuse together to form a combination nucleus. 



As soon as the fusion is complete the water collected beneath 

 the peristome of macrogamete is ejected, the protoplasm moves 

 forward to take its place, carrying with it not only the com- 

 bination nucleus but also the protoplasmic contents of the 

 microgamete. The latter is now reduced to a mere membranous 

 appendage, which shrivels up and eventually falls off and is lost. 



The combination nucleus soon elongates to form a fibrous 



