176 COMPARATIVE ANATOMY 



take place, producing eight daughter-cells, but more usually 

 when four cells are formed the maternal flagella are withdrawn, 

 each of the four cells acquire a new pair of flagella, the envelope 

 in which they are contained is dissolved, and they are set free 

 as four young Polytomse, each of which acquires a new envelope 

 on liberation. This process is repeated again and again for a 

 period of from four to six days, by which time the reproductive 

 activities seem to be exhausted and to require a stimulus for 

 their renewal. This stimulus is afforded by the act of conjuga- 

 tion. The Polytomse come together in pairs, each individual 

 of a pair constituting a gamete. The gametes fuse, nucleus 

 with nucleus and cell-body with cell-body, and form a spherical 

 zygote, which surrounds itself with a thick envelope and passes 

 into a resting condition. After a time the zygote divides into 

 two, each of the two again divides, and the four cells thus pro- 

 duced divide once again, so that eight are formed within the cyst 

 wall. Each of the eight cells develops a pair of flagella, the 

 thick cyst wall is dissolved, and the eight young Polytomae 

 emerge to resume an active existence. They divide, usually 

 into four, in the free-swimming condition, and the life cycle is 

 repeated. Occasionally a free swimming Polytoma will throw 

 off its flagella, secrete a thick envelope, and pass into the resting 

 stage without conjugation. In this case no division of the 

 contents of the cyst has been observed. 



As has just been remarked, the reproductive power seems 

 to become exhausted after a period of repeated multiplication 

 by division. We shall see in the next chapter that this is a 

 very common phenomenon amongst the Protozoa, yet it cannot 

 be called a universal one, since we have been unable to record 

 any exhaustion following upon repeated binary fission, either 

 in Amoeba or in Euglena. In neither of these forms does it 

 appear to be necessary that at regularly recurring periods 

 individuals should conjugate to form zygotes which, after a 

 period of rest, enter upon a new lease of reproductive life. 

 Yet the phenomenon of conjugation is so common both in 

 the lower plants and animals that we are tempted to believe 

 that it is a very necessary condition of existence and that it 

 has simply been missed, that we have failed to observe it, in 

 the forms in which it is recorded. We have seen that it is 

 of normal occurrence in Monocystis, that the nuclei of the 

 cysts of Actinosphaerium conjugate in a very peculiar way in 



