. Animal biology. Zoology; Biology. 76 PROTOZOA to fertilization in higher animals which possess sex, the smaller micronu- cleus being viewed as male and the larger one as female. However, it cannot, properly speaking, be called fertilization, since no gametes are involved. The macronucleus begins to degenerate soon after the micro- nucleus leaves it, breaking up into fragments. The two animals now separate and the fragments of the macronuclei slowly disappear, their substance being dissolved in the endoplasm. The fusion micronucleus in each of the exconjugants divides by mitosis into two, the
. Animal biology. Zoology; Biology. 76 PROTOZOA to fertilization in higher animals which possess sex, the smaller micronu- cleus being viewed as male and the larger one as female. However, it cannot, properly speaking, be called fertilization, since no gametes are involved. The macronucleus begins to degenerate soon after the micro- nucleus leaves it, breaking up into fragments. The two animals now separate and the fragments of the macronuclei slowly disappear, their substance being dissolved in the endoplasm. The fusion micronucleus in each of the exconjugants divides by mitosis into two, these into four, and these into eight, all equal in size. Four of 1. Animal before endomixis begins , . _;_i__-^ 3. Micronuc-lei again divide 4. Six out of eight micronuclei disappear. Animal divides 5. Each animal with one micronucleus. 2. Micronuclei divide. Macronucleus disintegrates 6. Micronucleus of each divides 7. Micronuclei divide again. S. Two micronuclei in each animal become macronuclei 9. Micronuclei divide and animals divide 10. Four ordinary individuals Fig. 25.—Diagram illustrating endomixis in Paramecium aureha Mi'iller. (From Wood- ruff, "Animal Biology," by the courtesy of The Macmillan Company.) Large black bodies are macronuclei, small ones micronuclei, and small circles micronuclei which disappear. these then become larger and develop into macronuclei. Of the four remaining, three disappear. The fourth divides into two, and the animal divides, each of the two individuals formed having one micronucleus and two macronuclei. In each individual the micronucleus again divides and this is followed by fission, producing four animals each with one micro- nucleus and one macronucleus. This description applies to Paramecium caudatum; the steps in the process are variously modified in other species. The significance of conjugation is uncertain. Some investigators believe that after a long series of fissions the animals become senescent and conjugation serves
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