212 COMPARATIVE ANATOMY 



So long as it is swimming about freely the microgamete has a 

 single micronucleus of fusiform shape, measuring some 4-5 p 

 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 the 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 macro- 

 gamete 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 micronuclear 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 re- 

 maining 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 surviving corpuscle in the micro- 

 gamete. These corpuscles increase notably in size, the dividing 

 wall separating them disappears ; they come into contact, and, 

 on doing so, both divide mitotically. 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 ab- 

 sorbed 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. 



