But this remark leads us to consider a little more attentively the anatomical features presented by the human embryo. The gill-slits just mentioned occur on each side of the neck, and to them the arteries run in branching arches, as in a fish. This, in fact, is the stage through which the branchiæ of a fish are developed, and therefore in fish the slits remain open during life, while the so called “visceral arches” throw out filaments which receive the arterial branches coming from the aortic arches, and so become the organs of respiration, or branchiæ. But in all the other vertebrata (i.e. except fish and amphibia) the gill-slits do not develop branchiæ, become closed (with the frequent exception of the first), and so never subserve the function of respiration. Or, as Mr. Darwin states it, “At this period the arteries run in arch-like branches, as if to carry the blood to branchiæ which are not present in the higher vertebrata, though the slits on the sides of the neck still remain, marking their former position.”
The heart is at first a simple pulsating vessel, like the heart of the lowest fishes, and the excreta are voided through a common cloacal passage—an anatomical feature so characteristic of the lower vertebrata, that it occurs in no fully formed member of the mammalian group, with the exception of the bird-like order of monotremata, which takes its name from presenting so striking a peculiarity.
At a later period the human embryo is provided with a very conspicuous tail, which is considerably longer than the rudimentary legs occurring at that period of development, and which Professor Turner has found to be provided with muscles—the extensor, which is so largely developed in many animals, being especially well marked.
Again, as Mr. Darwin says, “In the embryos of all air-breathing vertebrates, certain glands, called the corpora Wolffiana, correspond with and act like the kidneys of mature fishes;” and during the sixth month the whole body is covered very thickly with wool-like hair—even the forehead and ears being closely coated; but it is, as Mr. Darwin observes, “a significant fact that the palms of the hands and the soles of the feet are quite naked, like the inferior surfaces of all four extremities in most of the lower animals,” including monkeys.
Lastly, Professor Wyman has found that in a human embryo about an inch in length, “the great toe was shorter than the others; and, instead of being parallel to them, projected at an angle from the side of the foot, thus corresponding with the permanent condition of this part in the quadrumana.”[1]
Therefore, on the whole, we may conclude these brief remarks on embryology with the words of Professor Huxley:—“Without question, the mode of origin, and the early stages of the development of man, are identical with those of the animals immediately below him in the scale; without a doubt, in these respects he is far nearer to apes than the apes are to the dog.”[2]
[1] Proc. Amer. Acad. Scs., vol. iv., 1860, p. 17. It should be added, however, that although the direction taken by the great toe of man at this early age is doubtless, as Prof. Wyman states, more like that which obtains in the quadrumana, there is a slight anatomical difference in the mode of its articulation with the foot, which seems to assist in securing the forward direction taken by it in later life.
[2] Man's Place in Nature, p. 65.