“If we trace upwards the evolution of the vegetable kingdom, we find an important alteration in the relative magnitude of the sexual and asexual generations. Low down in the vegetable kingdom the great bulk of the species is constituted by asexual generation, and very little by the other. But vegetable evolution has been accompanied by a gradual increase of the asexual generation, together with a reduction in the importance of the sexual generation. Hence, in the higher plants almost everything that we notice is the asexual generation. This is to say that the fertilized egg, in the cells of which the full number of chromosomes has been restored, gives rise to the flowering plant with which we are so familiar. On the other hand, the sexual generation is merely represented by the spore stage of the plant, in which the number of chromosomes is reduced, and which has no free existence, but remains enclosed in the tissues of the flowering plant or asexual generation until fertilization occurs. This, of course, doubles the number of chromosomes, and initiates the asexual generation again.

“Now what is the case of the animal? In it the fertilized egg gives rise, as Dr. Beard has taught us, to the trophoblast, which is therefore the analogue of the flowering plant, though it is as insignificant as the flowering plant is conspicuous. This trophoblast has the characteristic of yielding germ-cells, and on occasion these may actually show ripening and reduction of the chromosomes, as observed by Prof. Farmer and his associates in the case of cancer. Normal trophoblast, however, yields many primary germ-cells, one of which becomes the embryo, whilst the rest, as we are about to see, are included within the embryo — most of them in one special spot, but some in abnormal situations. Occasionally two primary germ-cells may develop completely and independently, and the result is the production of what are called identical twins, two individuals, invariably of the same sex, and extraordinarily similar in physical characters.”