130 THE ENZYME TREATMENT OF CANCER
of the same class, but of a different order in other and ordinal characters. Immediately before this period is reached it begins to put on generic and specific characters, and thus it then begins to differ from all other embryos in these.” In other words, the embryo then first asserts its presence, announces its own individuality. It is then first present as a complete thing. It then first begins to use its own digestive apparatus, especially its pancreas gland, and in a higher mammal to feed itself by means of the allantoic placenta. This critical period is common to all backboned animals in their development. At this period the average marsupial is born into the world, and then it first begins its long mammary nutrition. In so great a hurry is it to get into the world that it forms its anus in the act of being born. The human embryo does the same at the like period, in the seventh week of gestation, as though it were a marsupial, although it has no use for this aperture for many months to come. Then the allantoic placenta—an organ of the embryo or sexual generation, like the pancreas gland—first begins to function, and then normally the trophoblast begins to fade, to be suppressed, and to degenerate.
Though ferments first made their appearance in my published writings in 1892—for I pride myself on having been one of the very few pupils the late Professor C. F. W. Krukenberg ever had—it was not until 1904 that their all-important bearings upon the critical period were evident. In human gestation, if at the critical period the embryo be wanting or very abnormal (a very abnormal human embryo can only persist as one of identical twins), the phenomena of the critical period are lacking, and the normal trophoblast, which always begins its life by eroding the uterine epithelium and wall, may go on with this process, exhibit indefinite powers of growth, and eat