Amidst all the infinitely rich and interesting material which lies before us in the ontogeny of vertebrate animals, that is, in the history of their individual development, I shall here confine myself to showing some of those facts which are of the greatest importance to the Theory of Descent in general, as well as in its special application to man. Man is at the beginning of his individual existence a simple egg, a single little cell, just the same as every animal organism which originates by sexual generation. The human egg is essentially the same as that of all other mammals, and cannot be distinguished from the egg of the higher mammals. The egg represented in Fig. 5 might be that of a man or an ape as well as of a dog, a horse, or any other mammal. Not only the form and structure, but even the size of the egg in most mammals is the same as in man, namely, about the 120th part of an inch in diameter, so that the egg under favorable circumstances, with the naked eye, can just be perceived as a small speck. The differences which really exist between the eggs of different mammals and that of man do not consist in the form, but in the chemical mixture, in the molecular composition of the albuminous combination of carbon, of which the egg essentially consists. These minute individual differences of all eggs, which depend upon indirect or potential adaptation (and especially upon the law of individual adaptation), are indeed not directly perceptible to the exceedingly imperfect senses of man, but are cognisable through indirect means, as the primary causes of the difference of all individuals. [Illustration: FIG. 5.—The human egg a hundred times enlarged. _a._ The kernel speck, or nucleolus (the so-called germinal spot of the egg). _b._ Kernel, or nucleus (the so-called germinal vesicle of the egg). _c._ Cell-substance, or protoplasm (so-called yolk of the egg). _d._ Cell-membrane (the yolk-membrane of the egg; in mammals, on account of its transparency, called zona pellucida). The eggs of other mammals are of the same form.]