The round speck which is the germ becomes a hollow sphere, a sort of sac with a double wall, which is known as the gastrula and whose orifice of invagination, when it closes, receives the name of the blastopore. This is protozoic life, the as yet gelatinous beginning of animal life, and is followed, after transformations that would take too long to enumerate, by polypoid life. Next, on either side of the head, appear the branchial arches, corresponding with the gills of the fish. At the end of the first month, the limbs are still no more than mere buds; on the other hand, the embryo is provided with a tail, which, folded against the body, nearly touches the forehead. It now has the appearance of a tadpole and lives a life which is wholly aquatic, bathed in the amniotic fluid which represents for it the water in which the embryos of fish and frogs move about freely. It now becomes a matter of forming a resolution and knowing what to do with it. The embryo is almost in the situation occupied by life at the origin of the species; and nature, as though to humiliate man or to humiliate herself by remembering her mistakes and hesitations, returns to her gropings, her asymmetrics, her repentances, her unsuccessful experiments. Tentative forms, such as the dorsal cord, are reabsorbed; the primitive kidneys disappear, to make room for the final kidneys, which are enormous, filling the greater portion of the peritoneal cavity. Enormous too is the liver, which invades almost the whole of the visceral cavity; enormous the head, almost as large as the rest of the body; and in this enormous head the primitive ocular vesicles are formed, themselves enormous, as is the umbilical vesicle. This is the incoherent and monstrous period corresponding with the period of madness and gigantism when nature, as yet inexperienced, was blindly sketching uncertain creatures, formidable, unbalanced and anomalous, birds, crocodiles, elephants and fish in one, as though she had not as yet decided what to do, not yet completed her classifications, disentangled her laws, or acquired the sense of proportion, of balance, or of conditions essential to the maintenance of the life which she was creating. 2 This, roughly speaking, is the recapitulation which occurs before our eyes, but of which, no doubt, many incidents escape us or do not sufficiently attract our attention, for it is possible that they reproduce types with which we are not acquainted and which have not even left geological traces, seeing that the number of species which have disappeared is infinitely greater than that of the species which we know.