amœba to be capable, is the fundamental fact in the functions of the fully developed muscle, nerve, and brain of the highest organisms.
The embryon, in its condition of a three-layered sac, soon begins to show a slight bilateral symmetry, and a chorda dorsalis appears. Its rank, as a vertebrate, is thus established in the dawning of that important structure, a backbone.
Allusion has been made to the ascidian as introducing the vertebrate type. Whatever may be thought of the claims of this animal to so important a place in the genealogical tree, there can be little doubt about the position of the amphioxus with its dorsal cord distinct and persistent throughout life. Though classed, on this account, among vertebrates, it is singularly wanting in vertebrate characteristics, having neither heart nor brain in the true sense of these words. It is also destitute of limbs, even of the most rudimentary kind, such as are found in the very lowest fishes. In fact, it is distinctly neither vertebrate nor invertebrate, thus admirably filling the position of a connecting link between these two great subdivisions of the animal kingdom.
At the chordonian stage of its development, the human embryon is equally destitute of a true heart, brain, and limbs, thus corresponding to a sub-type of the vertebrates called by Haeckel, Acrania, of which the amphioxus is the best-known representative. There is, nevertheless, in this heartless, brainless, limbless, and almost shapeless mass of but slightly differentiated protoplasm, that wonderful impulse of evolution by which its destiny, as an individual of the highest organic rank, is assured.
Along the line of the chorda dorsalis, rudimentary nerve-centers and spinal vertebræ gradually appear, the embryon thus entering on a grade of development comparable to that of the lowest fishes, in which the spinal column is cartilaginous rather than bony.
The budding limbs resemble budding fins; arches similar to those which, in water-breathing animals, support the gills are seen; and the rudimentary lungs are mere air-bladders.
Next arises the amnion stage, so named from an important though temporary nutritive organ whose development begins at this period; it is an extension of the yolk-sac, and contains a highly nutritious fluid.
The gill-arches gradually disappear, developing into more advanced structures; the heart becomes subdivided into four chambers; the air-bladders give place to true lungs; and, with the complete formation of a placenta, the mammalian stage of development is fully established. The embryon is henceforth recognizable as belonging to the class mammalia, the highest of the vertebrates.
As the growing organism becomes more and more complex, its progress is more and more gradual. We have seen how the germ passes, almost at a single step, from the gastrula to the rudimentary