THE ASYMMETRY OF THE CYCLE OF LIFE 161
seem to resemble the asexual generation of animals, rather than that of plants. In given circumstances many of them can attack the living compounds of the sexual generations of animals, or of the asexual generations of plants, pulling them down and building up in an opposite direction. As the plants and the animals are not genetically connected, so also, in all probability, the bacteria and the fungi have no genetic relationships with each other, or with the animals or the plants. Like the two latter, they are separate evolutions.
In 1889, in his study of the placentation of the hedgehog, Erinaceus europaeus, Professor A. A. W. Hubrecht* set lip the term “ trophoblast” (p. 298), at the same time assigning to it, as the name implies, a nutritive significance. The nutritive import of the trophoblast of normal mammalian gestation has since that time been confirmed by many other embryologists, notably by Professors E. van Beneden and M. Duval, and it has been “ generally accepted.” In the light of our present knowledge, a significance different from that seen in it by Professor Hubrecht must be recognized in “ trophoblast.” Trophoblast has, and can have, no nutritive import for the developing embryo. This is quite obvious, once it is noted that the natural compounds formed in it are built up in the wrong direction to be useful as food for the developing sexual organism. The term, therefore, cannot be employed in future in a physiological sense. As Duclaux said: “Nature alone knows how to manufacture the one isomer without producing the other.” The chemist in the laboratory manufactures equal amounts of both isomers. May one deny Nature the power to do the like
* Hubrecht. A. A. W.:” The Placentation of Erinaceus europaeus,” Quart. Journ. Micros. Sci., 1889, vol. XXX., pp.283-404. 13 plates.