612 



FAMILY: PROWAZEKELLID^ 



about 10 microns (Fig. 253). The writer (1921) has produced some 

 evidence that two of the ovoid forms become encysted together, and that 

 syngamy, with complete union of the cytoplasm and nuclei, follows 

 (Fig. 254, n-s). A vacuole now appears in the cytoplasm, and the single 

 nucleus begins to divide. The vacuole increases in size till the cytoplasm 



Fig. 252. — Prowazelcella lacence ( x 2,300). (After Belak, 1921.) 



A-B. Two types of flagellate, showing nucleus, parabasal body, and flagellar connections. 

 C-F. Stages in division. 



is reduced to a thin layer lining the cyst. At the two-nuclear stage the 

 nuclei lie at opposite poles of the cyst, which has a large central vacuole 

 and bears a close resemblance to Blastocystis, with which it has been 

 compared. Repeated divisions of the nuclei take place, while the cyst 

 increases in size till it may reach a diameter of about 70 microns. At this 



