VORTICELLA 1 41 



are discharged into the vestibule by an anus, which in some 

 species is a permanent opening through the ectoplasm. 

 The reproduction of Vorticella takes place by binary 

 fission, which is of two kinds — ordinary fission, 



epro uc on. ^^ t k at w hi c h forms conjugants. In ordinary 

 fission, the rim closes in over the disc, the body becomes 

 shorter and wider, and the meganucleus contracts and lies 

 across the body, which then divides into two, the plane of 

 fission being in line with the stalk. The nuclei behave as 

 in Paramecium. One of the daughters remains upon the 

 stalk; the other grows a circlet of cilia in the hinder 

 region, at the level at which the ectoplasm thickens, breaks 

 off, and swims away by means of its cilia, to settle down 

 elsewhere by the end which was attached to the stalk of 

 the parent. It grows a new stalk for itself. In this form 

 of reproduction the offspring are equal in bulk. In the 

 fission which forms conjugants the parent gives rise to one 

 large individual and one or more of a smaller size. The 

 small individuals may arise by unequal binary fission, 

 sometimes called budding, or by equal fission, followed by 

 division of one product into four by repeated fission. 1 In 

 either case the small individuals resemble the free product 

 of ordinary fission in all but size. 



The small individuals thus formed swim away, and each 

 attaches itself by its hinder end to the lower 



onjuga on. ^^ ^ ^ body f one f jjjg s t a lked indi- 

 viduals. Most of the organs of the small individual now 

 disappear, and the ectoplasm between the two conjugants 

 is absorbed into their endoplasm, which becomes continuous. 

 The meganucleus in each begins to break up and disappear. 

 Meanwhile the micronucleus of the small conjugant has 

 divided into two. Now the micronuclei of both conjugants 

 divide twice, so that the larger contains four and the 

 smaller eight micronuclei. In each case all but one of 

 these perish and the survivor divides into two, which corre- 

 spond to the male and female pronuclei of Paramecium. 

 This division takes place while the two micronuclei are 



1 The various kinds of fission of Amaba and animals related to it 

 (Protozoa, p. 143) may be classed as : (1) equal binary fission (p. 118), 

 (2) budding (p. 141), (3) repeated fission (p. 123), (4) multiple 

 fission (p. 118). 



