. The chordates. Chordata. Reproductio 237. Fig. 201. Human ovum surrounded by follicular cells. Actual diameter of ovum about mm. (C) Cytoplasm containing some yolk; (CR) corona radiata; (F) follicular cells; (N) nu- cleus; (ZP) zona pellucida. (After Nagel. Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The Rlakiston Company.) sperm chromatin becomes joined with the chromatin of the egg nucleus. The "maturation" process through which all germ-cells pass reduces their chromatin to approximately half that contained in body cells, so that the union of sperm
. The chordates. Chordata. Reproductio 237. Fig. 201. Human ovum surrounded by follicular cells. Actual diameter of ovum about mm. (C) Cytoplasm containing some yolk; (CR) corona radiata; (F) follicular cells; (N) nu- cleus; (ZP) zona pellucida. (After Nagel. Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The Rlakiston Company.) sperm chromatin becomes joined with the chromatin of the egg nucleus. The "maturation" process through which all germ-cells pass reduces their chromatin to approximately half that contained in body cells, so that the union of sperm chro- matin and egg chromatin provides the fertilized egg with a nucleus containing the full complement of chromatic bodies (chromosomes) characteristic of all body cells of the animal. Therefore the fertilized egg, although the product of two cells, possesses the mechanism of a single cell. It possesses no visible structures which would adequately account for its development into a large complex animal like the par- ent animals. Compared to such cells as those of muscle and ner- vous tissue, it is strikingly devoid of visible special mechanism. Yolk is characteristic of eggs, but yolk is an inert food substance, not a mechanism. Developmental Potentialities. The motile and aggressive sper- matozoon might seem to be the essentially "animal" body in develop- ment, while the relatively large unfertilized egg, burdened with inert yolk, would appear rather as a passive and vegetative thing. But in normal development the spermatozoon merely imparts the stimulus which initiates development and provides for inheritance from a male parent. Experimentation has proved that artificially applied mechan- ical or chemical agencies may stimulate an egg to develop and produce a characteristic adult without the assistance of a spermatozoon. Obviously, however, such an adult inherits only from a mother, since its "father" may have been a mere needle-prick or a change in osmot
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