Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 Fro. 101.—Ovum of Neplielis (after O. Hertwig). a, the ovum half-an-hour after deposition. a projection of the protoplasm indicates the commencing- formation of the first polar body ; the nuclear spindle is visible. 6, The same an hour later, with polar body extruded, and after entrance of the spermatozoon. Sk, male pronucleus. c, The same another hour later without egg membrane, and with two polar bodies and male pronucleus (Sk); d, the same an hour later with approximated female and ma
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 Fro. 101.—Ovum of Neplielis (after O. Hertwig). a, the ovum half-an-hour after deposition. a projection of the protoplasm indicates the commencing- formation of the first polar body ; the nuclear spindle is visible. 6, The same an hour later, with polar body extruded, and after entrance of the spermatozoon. Sk, male pronucleus. c, The same another hour later without egg membrane, and with two polar bodies and male pronucleus (Sk); d, the same an hour later with approximated female and male pronuclei; Ek, polar bodies. DEVELOPMENT. It follows from the facts of sexual reproduction that the simple cell must be regarded as the starting-point for the development of the organism. The contents of the ovum spontaneously or under the influence of fertilization enter upon a series of changes, the final result of which is the rudiment of the body of the embryo. These changes consist essentially in a process of cell division which implicates the whole protoplasm of the ovum, and is known, as segmentation.
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