CCEXOBIEyE 



;oi 



having in the meantime developed two cilia from each of its cells.. 

 Sexual reproduction takes place in the following way. Sixteen daughter- 

 families are first of all formed in the same manner, but the gelatinous 

 envelopes of the young colonies deliquesce, and the separate 256 (= 16 

 X 16) swarm-cells are set free as zoogametes. These vary in size, but 

 are always rounded and green at the posterior end, pointed hyaline and 

 with a red pigment-spot in front, where they bear two cilia. Among 

 the crowd of these swarm-cells — now swimming about freely — some, 

 irrespective of their relative size, approach one another in pairs, their 

 pointed anterior apices coming into contact, and they finally coalesce 

 into a body which has at first somewhat the shape of an hour-glass, but 

 gradually contracts into a sphere, in which the two pigment-spots and 

 the four cilia are still to be seen for a time, but soon disappear. This 

 whole process occupies about five minutes. The resulting zygosperm 

 is then a spherical cell enclosed in a cell-wall, which remains at rest for 

 some time as a hypnosperm, its green colour becoming changed to 

 brick-red. If the dried-up hypnosperms are placed in water, they begin 

 to germinate after about twenty-four hours. The outer layer of the 

 cell-wall is ruptured, an inner layer becomes gelatinous and swells up, 

 and the protoplasmic contents escape in the form of one, two, or three 

 large zoospores. Each of these, after a short period of swarming, loses 

 its cilia, surrounds itself with a 

 gelatinous envelope, and breaks 

 up, by successive bipartitions, 

 into sixteen portions, which 

 develop cilia, and form them- 

 selves into a new coenobe. 



A still more remarkable 

 succession of phenomena is 

 exhibited by Stephanosphcera 

 Cohn, a rare organism occur- 

 ring occasionally in the rain- 

 water which collects in the 

 hollows of large stones in 

 mountainous countries. In 

 addition to the process of 

 vegetative or non-sexual pro- 

 pagation, the cells belonging to a family, each of which possesses a red 

 pigment-spot, divide repeatedly into zoogametes, which ultimately 

 become free, and coalesce into resting zygosperms or hypnosperms. 

 Motionless balls, which are probably the result of this conjugation, 

 accumulate at the bottom of the water, and assume a red colour. After- 



Fig. 262.— Pandorlna iitoj-iiin Fhrb. a, swarming 

 coenobe ; b, c, swarm-cells ; d, e, comugation of 

 zoogametes ; _/, z^'gosperm ( x 500). (After Prings- 

 heim.) 



