. Darwin, and after Darwin. An exposition of the Darwinian theory and a discussion of post-Darwinian questions. Evolution; Heredity. Embryology. 121 the zona pelhicida, which contains—first, the proto- plasmic cell-substance or "yolk," within which lies, second, the nucleus or germinal vesicle, within which again lies, third, the nucleolus or germinal spot. This description is true of the egg-cells of all animals, if we add that in the case of the lowest animals—such as sponges, &c.—there is no enveloping membrane: the egg-cell is here a naked cell, and its constituent protoplasm
. Darwin, and after Darwin. An exposition of the Darwinian theory and a discussion of post-Darwinian questions. Evolution; Heredity. Embryology. 121 the zona pelhicida, which contains—first, the proto- plasmic cell-substance or "yolk," within which lies, second, the nucleus or germinal vesicle, within which again lies, third, the nucleolus or germinal spot. This description is true of the egg-cells of all animals, if we add that in the case of the lowest animals—such as sponges, &c.—there is no enveloping membrane: the egg-cell is here a naked cell, and its constituent protoplasm, being thus unconfined, is free to perform protoplasmic movements which it does after the. Pig. 30.—Ovarian OTum of a Mammal, 'a) magnified and viewed under pressure, {b) burst by increased pressure, with yolk and nucleus escaping : {c') the nucleus more freed from yolk-substance. (From Quaitis AnatoJiiy, after Allen Thomson.) manner, and with all the activity, of an amoeba. But even with respect to this matter of an enveloping membrane, there is no essential difference between an ovum of the lowest and an ovum of the highest animals. For in their early stages of development within the ovary the ova of the highest animals are likewise in the condition of naked cells, exhibiting amoebiform movements; the enveloping membrane of an ovum being the product of a later 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 Romanes, George John, 1848-1894. Chicago The Open Court Pub. Co
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