204 COMPARATIVE ANATOMY 



under continuous observation. In a free swimming form the 

 nucleus divides mitotically, and a transverse constriction of the 

 protoplasmic body follows, not involving the envelope. The 

 two flagella of the parent form remain attached to the anterior 

 product of division and the organism continues to swim about 

 actively, consisting now of two cells lying inside a common 

 membranous envelope, the more anterior of the two cells 

 alone bearing flagella. The next division is at right angles 

 to the first, and therefore parallel to the long axis of the 

 original parent cell. Four cells are thus produced, still 

 enclosed within the envelope of the original parent form, and 

 still the two flagella are active and attached to one only of 

 the four daughter-cells. Another division may take place, 

 producing eight daughter-cells, but more usually when four 

 cells are formed the maternal flagella are withdrawn, each of 

 the four cells acquires a new pair of flagella, the envelope 

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

 free as four young Polytomae, 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 conjugation. The Polytomae 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 produced divide once again, so that eight 

 cells 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 -a resting stage without conjugation. 

 In this case no division of the contents of the cyst has been 

 observed. 



In both Polytoma and Copromonas, the reproductive power 

 seems to become exhausted after a period of repeated multi- 

 plication by division. We shall see in the next chapter that 



