224 FERTILIZATION OF THE OVUM 



gation counteracts the tendency to senile degeneration and causes 

 rejuvenescence, as maintained by Biitschli and Minot. 1 



In Stylonychia pustulata, which Maupas followed continuously from the end of 

 February until July, the first conjugation occurred on April 2Qth, after 128 bi-parti- 

 tions; and the epidemic reached its height three weeks later, after 175 bi-partitions. 

 The descendants of individuals prevented from conjugation died out through " senile 

 degeneracy,' 1 after 316 bi-partitions. Similar facts were .observed in many other 

 forms. The degeneracy is manifested by a very marked reduction in size, a partial 

 atrophy of the cilia, and especially by a more or less complete degradation of the 

 nuclear apparatus. In Stylonychia pustulata and Onychodronms grandis this process 

 especially affects the micronucleus, which atrophies, and finally disappears, though 

 the animals still actively swim, and for a time divide. Later, the macronucleus 

 becomes irregular, and sometimes breaks up into smaller bodies. In other cases, 

 the degeneration first affects the macronucleus, which may lose its chromatin, 

 undergo fatty degeneration, and may finally disappear altogether {Stylonychia 

 mytilus), after which the micronucleus soon degenerates more or less completely, 

 and the race dies. It is a very significant fact that toward the end of the cycle, as 

 the nuclei degenerate, the animals become incapable of taking food and of growth ; 

 and it is probable, as Maupas points out, that the degeneration of the cytoplasmic 

 organs is due to disturbances in nutrition caused by the degeneration of the nucleus. 



The more essential phenomena occurring during conjugation are 

 as follows. The Infusoria possess two kinds of nuclei, a large 

 macronucleus and one or more small micronuclei. During conjuga- 

 tion the macronucleus degenerates and disappears, and the micronu- 

 cleus alone is concerned in the essential part of the process. The 

 latter divides several times, one of the products, the germ-nucleus, 

 conjugating with a corresponding germ-nucleus from the other indi- 

 vidual, while the others degenerate as "corpuscules de rebut." The 

 dual nucleus thus formed, which corresponds with the cleavage- 

 nucleus of the ovum, then gives rise by division to both macronuclei 

 and micronuclei of the offspring of the conjugating animals (Fig. 109). 



These facts may be illustrated by the conjugation of Paramcecium 

 caudatum, which possesses a single macronucleus and micronucleus, 

 and in which conjugation is temporary and fertilization mutual. The 

 two animals become united by their ventral sides and the macronu- 

 cleus of each begins to degenerate, while the micronucleus divides 

 twice to form four spindle-shaped bodies (Fig. no, A, B\ Three of 

 these degenerate, forming the "corpuscules de rebut," which play 

 no further part. The fourth divides into two, one of which, the 

 " female pronucleus," remains in the body, while the other, or "male 

 pronucleus," passes into the other animal and fuses with the female 

 pronucleus (Fig. no, C-H). Each animal now contains a cleavage- 

 nucleus equally derived from both the conjugating animals, and the 

 latter soon separate. The cleavage-nucleus in each divides three 



i Cf. p. 179. 



