SEX IN PROTOZOA 163 



formed, ;ind thus reduction is accomplished by a single nuclear divi- 

 sion (7). Ihc new asexual cells begin to develop flagellar bands, but 

 division is rapid for 2 or 3 days, so that the flagellar bands may not 

 attain full length until after this period of rapid division has ended. 



Urhiynipbii is a hypermastigote with two anterior groups of fla- 

 gella symmetrically placed opposite each other. Besides the flagella 

 there are numerous parabasal bodies and very slender axostyles on 

 eacii side (Fig. L, 1). It has a diploid nucleus. Its sexual cycle has 

 recently been described by Cleveland (1951b). 



In ordinary mitosis no organelles are lost. The two centrioles 

 elongate and produce a spindle at their distal ends. The two flagellar 

 areas separate and go to the daughter cells, a new group of anterior 

 flagella and other organelles developing to match the group passed on 

 from the parent. The sixteen chromosomes divide much as in other 

 hypermastigotes. 



iMeiosis, which is pregametic, begins 5 to 6 days before the roach 

 molts and takes less than a day for completion, in most cases only a 

 few hours. The achromatic figure forms and functions much as it 

 does in ordinary mitosis, but the other organelles behave differently. 

 The flagellated areas with most of the parabasals and axostyles move 

 away from the nucleus with which the nuclear sleeve and achromatic 

 figure remain (Fig. L, 2), The old flagellar apparatus remains for 

 some time, undergoing various forms of disruption, and does not 

 disappear until after the new flagellated areas are formed in the zy- 

 gote. Meanwhile the nucleus undergoes a single meiotic division, the 

 chromosomes pairing and segregating without dividing, producing 

 haploid pronuclei (3). The cell body does not divide, but autogamous 

 pronuclear fusion takes place very soon after the haploid nuclei are 

 formed. 



During autogamous nuclear fusion, one centriole, with a rem- 

 nant of the nuclear sleeve, moves away and degenerates. After nuclear 



Fig. K. Leptospirovyi/ipha zvachula, inm-i Cleveland (1951a). 



1, agamont; 2, late stage in formation of gametocvte, spiraling portion of flagellar 

 bands in process of disintegration; 3, early anaphase of gametocyte, each group of 

 sister chromatids forms a partial ring and rings lie close together, those of one 

 ring slightly darker than those of the other; 4, male gamete entering female gamete, 

 the male is losing its extranuclear organelles; 5, flattening of juxtaposed membranes 

 of fusing pronuclei; 6, nuclear fusion completed; 7, anaphase of one-division zygotic 

 meiosis, cell has increased greatly in size. 



