General physiology; an outline of the science of life . During conjugation the macronucleus perishes,disintegrating and dissolving in the protoplasm. If the Paramce-cium be a form possessing one micronucleus, such as Parametriumcaudatum, where the relations are simplest, the micronucleus ineach individual divides twice in succession, so that four partialnuclei arise. Three of these likewise dissolve in the protoplasm, 202 GENERAL PHYSIOLOGY but the fourth divides once more in each individual, and onehalf (the male nucleus) passes over the protoplasmic bridgeinto the other individual, so that e
General physiology; an outline of the science of life . During conjugation the macronucleus perishes,disintegrating and dissolving in the protoplasm. If the Paramce-cium be a form possessing one micronucleus, such as Parametriumcaudatum, where the relations are simplest, the micronucleus ineach individual divides twice in succession, so that four partialnuclei arise. Three of these likewise dissolve in the protoplasm, 202 GENERAL PHYSIOLOGY but the fourth divides once more in each individual, and onehalf (the male nucleus) passes over the protoplasmic bridgeinto the other individual, so that each one of the pair now con-tains a female nucleus of its own, and a male nucleus fromthe other. These two nuclei immediately fuse together and thendivide, one half becoming a new macronucleus, and the other halfa new micronucleus. After such a mutual exchange of half-nuclei,the pair separate again and the conjugation is ended. The phenomena of fertilisation in sexual reproduction are derivedphylogenetically from the conjugation of asexual unicellular. Fig. 83.—Fertilisation of the ovum of the thread-worm (Ascaris megalocephala) in six successivestages. The maturation of the ovum, , the extrusion of the polar bodies, takes place simul-taneously. (After O. Hertwig.) organisms; essentially the same facts are found in the former asin the latter. The process of fertilisation is not entirely the samein different species; at least in the two species that thus far havebeen most fully investigated, the egg of the sea-urchin and that ofthe thread-worm of the horse, some slight differences have beenobserved, although the essential factors agree throughout. We shall consider, first, the fertilisation of the ovum of the thread-worm. The maturation of the ovum, , the extrusion of thepolar bodies, takes place while the sperm-cell is entering the the latter process is taking place (Fig. 83, /), the egg-nucleus, which up to this time has lain in the middle of the egg, ELEME
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