. The doctrine of descent and Darwinism. Evolution. GERM-CELLS. 45 In manyanlmals it is provided with a sheath or membrane of its own ; in others it remains naked, and in that case frequently displays the remarkable movements of pro- toplasm. The germ-cells of different classes of animals vary considerably in their microscopic dimensions ; nevertheless, in the whole animal kingdom, from the sponges and polypes up to the mammals inclusive of man, they are essentially similar. Nor do non-essential differences appear until the primitive germ-cell is more abundantly provided with yelk and albumen,
. The doctrine of descent and Darwinism. Evolution. GERM-CELLS. 45 In manyanlmals it is provided with a sheath or membrane of its own ; in others it remains naked, and in that case frequently displays the remarkable movements of pro- toplasm. The germ-cells of different classes of animals vary considerably in their microscopic dimensions ; nevertheless, in the whole animal kingdom, from the sponges and polypes up to the mammals inclusive of man, they are essentially similar. Nor do non-essential differences appear until the primitive germ-cell is more abundantly provided with yelk and albumen, and has surrounded itself with a specially thick and perforated shell, as in insects and fishes, or with a peculiarly formed sheath, in the shape of a double concave lens, as, for instance,* in some Turbellaria. As a rule, the ova are formed in special organs, the ovaries. The other sexual element, the sperm, contains, as its peculiar active constituents, the spermatozoa (fig. 4 s), which consist of a pointed, elliptic, or occasionally of a hook-shaped, head, and a thread-like body. As long as the sperm is capable of fecundation, the filamentous appendage performs serpentine movements, and the development of the spermatozoa from cells, as well as the comparison of their movements with the vibrating movements of ciliated and flagellate cells, enable us to recognize them also as modified cell structures. The vehement dispute of last century between Evo- lutionists and Epigenists has now a merely historical interest. The former maintained that either in the ovum or in the sperm-corpuscle the whole future organism. 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 Schmidt, Dr. (Eduard Oskar), 1823-1886. New York, D. Appleton
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