Walther, in 1808, said: “The human fœtus passes through its metamorphosis in the cavity of the uterus in such a way that it repeats all classes of animals, but, remaining permanently in none, develops more and more into the innate human form. First the embryo has the form of a worm. It reaches the insect stage just before its metamorphosis. The origin of the liver, the appearance of the different secretions, etc., show clearly an advance from the class of the worm into that of the mollusk.” Meckel first in 1808, again in 1811, and more fully in 1821 made much more definite comparisons between the embryos of higher forms and the adult stages of lower groups. He held that the embryo of higher forms, before reaching its complete development, passes through many stages that correspond to those at which the lower animals appear to be checked through their whole life. In fact the embryos of higher animals, the mammals, and especially man, correspond in the form of their organs, in their number, position, and proportionate size to those of the animals standing below them. The skin is at first, and for a considerable period of embryonic life, soft, smooth, hairless, as in the zoophytes, medusæ, many worms, mollusks, fishes, and even in the lower amphibians. Then comes a period in which it becomes thicker and hairy, when it corresponds to the skin of the higher animals. It should be especially noted here, that the fœtus of the negro is more hairy than that of the European. The muscular system of the embryo, owing to its lack of union in the ventral wall, corresponds to the muscles of the shelled, headless mollusks, whose mantle is open in the same region. Meckel compares the bones of the higher vertebrates with the simpler bones of the lower forms, and even with the cartilages of the cephalopod. He points out that in the early human embryo the nerve cord extends the whole length of the spinal canal. He compares the simple heart of the embryo with that of worms, and a later stage, when two chambers are present, with that of the gasteropod mollusk. The circulation of the blood in the placenta recalls, he says, the circulation in the skin of the lower animals. The lobulated form of the kidney in the human embryo is compared with the adult condition in the fishes and amphibians. The internal position of the reproductive organs in the higher mammals recalls the permanent position of these organs in the lower animals. The posterior end of the body of the human embryo extends backwards as a tail which later disappears.