144 GENERAL ZOOLOGY 



stalk. If conditions become unfavorable everywhere, a 

 wall is formed around the body, and encystment takes place. 

 Vorticella can then remain in a quiescent condition until 

 better times come. 



Race Preservation. Vorticella can produce new individu- 

 als by fission. The trophonuclei and micronuclei elongate 

 and divide, the bell becomes wide, and a constriction ap- 

 pears at the distal end which splits the cell longitudinally 

 into two parts, each with a pair of nuclei. Two individuals 

 are thus formed, attached to one stalk. One of them soon 

 forms a second band of cilia about its body, breaks loose and 

 swims away to settle down and form a stalk of its own. Vor- 

 ticella may multiply for many generations by such binary 

 fission but will finally die of old age unless conjugation takes 

 place. 



For conjugation two kinds of individuals are formed, 

 macrogametes and microgametes. The former are stalked 

 individuals differing neither in size nor in any other obvious 

 feature from an ordinary Vorticella. The microgametes, 

 as their name implies, are much smaller. They are also 

 peculiar in possessing a posterior circle of cilia about the 

 body and in being without a stalk. A microgamete when 

 set free from the "parent" (by cell-division), swims about 

 until it finds a macrogamete and then conjugates. Con- 

 jugation begins at daybreak and lasts thirteen or fourteen 

 hours. The cytoplasm of the macrogamete fuses with that 

 of the microgamete. The trophonuclei in both cells then 

 degenerate and take no part in what follows. The micro- 

 nucleus in each cell, after dividing several times in order to 

 cast out some of the nuclear material, is ripe, and then fuses 

 with a nucleus formed from the micronucleus originally in 

 the other cell. Thus a single new cell, the zygote, origi- 

 nates from the macrogamete and the microgamete, which 

 were two separate cells at the beginning of conjugation. 



When compared with those in the amoeba, the conju- 

 gating cells in Vorticella show progress along the line of sex 

 differentiation. The small motile microgamete has male 



