. General physiology; an outline of the science of life. . FlG. 80.—Ova. /. Splierical ovum of a sea-urchin. (After Hertwig.) 11. Amceboid ovum of a calcareous sponge. (After Haeckel.) tremely tiny in comparison with them. The spermatozoa consist chiefly of nuclear substance, and have only a thin protoplasmic covering; in most cases the latter is extended into a motile flagellum, the tail, which is distinguished from the rest of the body, the head, and serves for the movement of the spermato- zoon in seeking the ovum. The finer structure of the sperm-cell, as the detailed investigations of Bal
. General physiology; an outline of the science of life. . FlG. 80.—Ova. /. Splierical ovum of a sea-urchin. (After Hertwig.) 11. Amceboid ovum of a calcareous sponge. (After Haeckel.) tremely tiny in comparison with them. The spermatozoa consist chiefly of nuclear substance, and have only a thin protoplasmic covering; in most cases the latter is extended into a motile flagellum, the tail, which is distinguished from the rest of the body, the head, and serves for the movement of the spermato- zoon in seeking the ovum. The finer structure of the sperm-cell, as the detailed investigations of Ballowitz ('90) have recently shown, is very complicated, and very various differentiations occur among different animals. The accompanying illustrations present some examples of this (Fig. 81). But both the spermatozoa and the ova are always complete cells, and contain both the essential cell-constituents, protoplasm and nucleus—a fact upon which special emphasis should be laid. Before fertilisation takes place, in some cases also during the beginning of fertilisation, there occurs the ^naturation of the ovum, which consists in the formation, by means of two successive divisions of the nucleus, of two buds, the pola7' bodies or direction-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Verworn, Max, 1863-1921; Lee, Frederic S. (Frederic Schiller), 1859-1939. London, Macmillan and co. , limited; New York, The Macmillan company
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