196 GENERAL EMBRYOLOGY 



micronucleus in each organism. Each larger or " stationary" micro- 

 nucleus remains passive, but the smaller or " migratory" nucleus becomes 

 active and moves through the bridge of fused cytoplasms to the sta- 

 tionary micronucleus of the other individual, with which it fuses forming 

 a single compound or zygotic nucleus in each individual. The two 

 Paramoecia now separate each with a nucleus of modified composition. 

 The macronucleus, which has taken no share in the events of fertilization, 

 now fragments and dissolves leaving the fusion nucleus as the only 

 nuclear structure present. Then by three successive divisions the 

 fusion nucleus gives rise to eight small nuclei ; four of these in the pos- 

 terior end of the cell, remain small, as micronuclei, while the other four, 

 in the anterior end, enlarge, forming macronuclei. During the first 

 fission of this cell each daughter cell receives two nuclei of each kind, 

 and at the next division each of the four granddaughters of the " zygote " 

 receives one micronucleus and one macronucleus, and the normal 

 vegetative condition is restored. This form of karyogamy is peculiar 

 for at least three reasons; the nuclei alone fuse, both of the gametes 

 undergo nuclear reconstruction, and the individuality of the gametes is 

 not lost. 



The sessile Ciliates show an adaptive modification of this process which 

 is anisogamic in character. In the common Vorticella, for example, 

 while one of the conjugants or gametes retains its normal vegetative 

 form, the other is small and one of a brood of four, which become free- 

 swimming. A small individual upon meeting a large one, is actually 

 absorbed by it. The early nuclear history of each organism is much the 

 same as in Paramcceium, save that the final micronucleus of the mega- 

 gamete is one of four, that of the microgamete one of eight, the remainder 

 in each gamete having degenerated. But after the equivalents of the 

 stationary and migratory micronuclei (idiochromidia) are formed 

 in each gamete, the process changes somewhat, for now one of the two 

 micronuclei (the equivalents of one stationary and one migratory body) 

 degenerates in each organism, while those remaining fuse together 

 forming thus only a single fusion nucleus in the single but duplex zygote. 

 Thus there is no mutual fertilization, and while strictly this is anisog- 

 amous, it is mentioned here because it is clearly derived from the more 

 typical Ciliate condition as an adaptation to the sessile life of the vege- 

 tative form. 



Coming now to anisogamous karyogamy (Fig. 103), we should note 

 that transitional conditions between isogamy and anisogamy are not 

 infrequent. Thus in the Flagellate, Bodo, the conjugants may be 

 either of equal or unequal size, apparently in an accidental fashion. 

 The colonial Pandorina forms gametes of three sizes, small, medium, 

 and large, and conjugation may occur between any smaller and any 

 larger individuals anisogamically, or the small or the medium organisms 



