PHENOMENA ACCOMPANYING FERTILIZATION 531 



each of which secretes a gelatinous envelope about itself. The 

 nucleus then divides by mitosis followed by division of the cell into 

 two daughter cells which he calls cytospores No. 2. The nuclei of 

 the latter undergo two successive "maturation" divisions resulting 

 in one pronucleus and two "polar bodies" in each (Fig. 221), the 

 latter degenerating and disappearing. The two cytospores of the 

 second order now unite again, reforming cytospore No. 1 and 

 fertilization is completed by fusion of the pronuclei (Fig. 221). 

 Belaf quite recently (1922) has given a more complete description of 

 the process in the allied form Actinophnjs sol. The individuals 

 draw in their pseudopodia, ordinary A'egetative division of the 

 nucleus follows, and the cell divides into two. By this division 

 which Belaf terms the "progamous" division, the two gametes 

 are formed and after each of them has undergone two meiotic 

 divisions of the nuclei they reunite to form the zygote. One of 

 them anticipates the other in these divisions and develops a pseudo- 

 podial process which the other lacks. By this process the first 

 fusion of the two cells takes place. The original cell thus is a 

 gamont and the fusing gametes are sister cells, one of which shows 

 an incipient sex difi'erence in its precocious acti\ity and by its 

 pseudopodium-like process. (Fig. 209, p. 501). there are 44 

 chromosomes in the vegetative mitoses of Actinoyhrys sol and after 

 the progamous di^•ision the gametic nuclei swell, chromosomes 

 arrange themselves in ])airs (parasynapsis) oriented towards one 

 pole of the nucleus. These double chromosomes shorten and 

 ultimately form the nuclear plate of the first meiotic spindle. Here 

 the two parts of the double chromosomes are separated and pass to 

 the resulting nuclei each of which thus has 22 single chromosomes. 

 A second meiotic division results in the longitudinal splitting of these 

 22 chromosomes so that the pronuclei and 'the two "polar bodies" 

 in each gamete have 22. One of the products of each division 

 degenerates and is absorbed in the cytoplasm, and these are com- 

 pared with the polar bodies in Metazoa. The two gametes then 

 fuse, their nuclei fuse and the zygote becomes encysted (Fig. 209). 

 In this case the chromosome cycle is remarkably similar to that of 

 chromosomes of the metazoan e^g and sperm in their maturation 

 divisions. 



Analogous processes may take place in other t\'pes of Protozoa 

 in which fusion of gametes occurs, but the chromosome history is 

 known in but few cases. In Gregarinida there are several pro- 

 gamous divisions of the gamonts the last of which according to 

 Mulsow's (1911) observations of J/o;?6r7/,?^/.s' rostratu being a reducing 

 division wdiereby the chromosomes are reduced in number from 

 8 to 4 (Fig. 63, p. 122). 



(c) Zygotic Meiosis (Wilson).— Keduction in number of chromo- 

 somes subsequent to miclear fusion of gametes occurs in rare 



