56 THE ENZYME TREATMENT OF CANCER
as such, normally gives birth to an embryo, and, as elsewhere indicated, the embryomata of Wilms arise, and must do so, from such primary germ-cells. This unfolding of a primary germ-cell is equivalent to its landing in a cul-de-sac ; its powers of growth and increase and its life are thereby limited. The contrast between this and the larva or asexual generation in these respects is very striking. The latter, like the corresponding generation in plants, often possesses indefinite unrestricted powers of growth in an apical fashion.* In many animals there is only one apical region of growth in the asexual generation; in the hydroid polypes there may be many such, and Weismann and De Vries have already noted their powers of indefinite unrestricted growth. That which brings to a sharp and sudden close the growth of the asexual generation, if there be but one growing point, is the cutting-off
* Of great, but hitherto unrecognized, importance in this direction, are certain results of experimental embryology—such as those of Driesch, Morgan, and others, upon echinoderms, and of E. B. Wilson and others upon Amphioxus, etc. (For a full account see E. B. Wilson’s “ The Cell,” second edition, 1900; or Korschelt und Heider, “Lehrbuch der vergleichende Entwicklungsgeschichte, Allgemeiner Teil,” Jena, Gustav Fischer, 1902; or J. W. Jenkinson, “Experimental Embryology,” Oxford, at the Clarendon Press, 1909). Space does not permit of a description and discussion of these experiments. The general result is, that in certain animals, if the cells of the early cleavage of the egg be separated, each of them will give rise to a diminutive larva, the size of this varying directly with the size of the original blastomere. These minute larvae are often, but erroneously, spoken of as “embryos.” Though they may go on living for a considerable time, no case is known in which an embryo with sexual organs arises upon such a larva, as would, of course, happen in a normal development. Allied to this latter circumstance is the fact that the in vertebrate larvae, obtained by embryologists by artificial fecundation, never give rise to the sexual form, no matter how long they may be kept living in the laboratory. The writer has reared Echinus (sea-urchin) larvae for fifteen days without the formation of a sea-urchin upon any one of them. Many other similar facts could be mentioned.