SEX IN PROTOZOA 165 



fusion the remaining ccntriolc divides, and from each of the daughter 

 centriolcs a new half of the flagellar apparatus, along with axostyles, 

 parabasals, and nuclear sleeve, develops. Even after the new flagellar 

 apparatus develops, the old flagellar areas are still persisting. This con- 

 dition lasts for about 2 days, and then "pseudoencystation" takes place 

 w itii a rounding up of the cell but without the formation of a cyst 

 membrane. At this time the old flagellar apparatus completely disap- 

 pears. This rounded-up condition lasts for about 2 days, then, shortly 

 after the host molts, the organisms resume normal activity. 



During these events a good many irregularities have been noted. 

 For example, when one of the centrioles fails to degenerate during 

 autogamic nuclear fusion, two complete sets of flagella and associated 

 structures develop. These tend to pull the cell into two parts, with 

 or without pulling the nucleus into two parts also. Anuclear and 

 partially nucleate cells die. 



Sometimes, instead of meiosis and autogamy, endomitosis may 

 take place producing a 4N or even an 8N nucleus. Usually centrioles 

 are also duplicated, such duplication leading to the production of 

 extra sets of flagella. These cells do not survive. Occasionally, instead 

 of a meiotic nuclear division, an ordinary mitosis takes place without 

 cell division. The two diploid nuclei so produced fuse to produce a 

 4iV nucleus. This raises the important question why pronuclei fuse. 

 Perhaps the gametogenic nuclear divisions produce physiological dif- 

 ferences in the nuclei which result in their fusion. Sometimes pro- 

 nuclei are cut into two parts by the spindle. In such cases the two 

 parts of the same nucleus do not fuse with each other but with the 

 other pronucleus, or with parts of it, if it has also been cleaved into 

 parts. Another type of anomaly is division of the centrioles, in which 

 case new sets of extranuclear organelles are produced but the nucleus 

 fails to divide. 



The very simple type of sexual cycle in Urinympha is consid- 

 ered by Cleveland in relation to other cycles from an evolutionary 



Fig. L. Urinympha talea, from Cleveland (1951b). 



1, diploid agamont in early mitotic prophase, showing two groups of flagella, para- 

 basals (pb), axostyles (ax), etc.; 2, entire anaphase of meiotic cell; eight chromosomes 

 are going to each pole; nucleus in center, with greatly disrupted flagellar areas, 

 parabasals and axostyles to the right; halves of nuclear sleeve almost separated; 3, 

 entire meiotic cell, discarded flagella, parabasals, and axostyles above the centrally 

 placed pronuclei. 



