PHENOMENA ACCOMPANYING FERTILIZATION 309 



chromosomes in the vegetative mitoses of Actinophrys sol and after 

 the progamous division the gametic nuclei swell, chromosomes 

 arrange themselves in pairs (parasynapsis) oriented toward one pole 

 of the nucleus. These double chromosomes shorten and ultimately 

 form the nuclear plate of the first meiotic spindle (Fig. 157). 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. 142). 

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





JK9H 



sm5 



am_ 



'Mm4m W ! ^W W&Wm 





sassgs*- v 



«i 







A 5 C 



Fig. 157. — Actinophrys sol. A, contraction of the double chromosomes of 

 strepsinine stage; B, metaphase of reduction division; C, anaphase of equation 

 division. X 1900. (After Belaf, Archiv f. Protistenkunde, 1926, courtesy of 

 G. Fischer.) 



chromosomes of the metazoan egg and sperm in their maturation 

 divisions. 



Analogous processes may take place in other types 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 Monocystis rostrata, being a reduc- 

 ing division whereby the chromosomes are reduced in number from 

 8 to 4 (Fig. 55, p. 101). Mulsow's interpretation is confirmed for 

 Monocystis by Calkins and Bowling (1920). 



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

 somes subsequent to nuclear fusion of gametes occurs in rare 

 instances but the phenomenon may be more widely spread than is 

 at present admitted. Two well authenticated cases are the coccidian 



