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In all the Craniotes the soft, indifferent cells of the mesoderm, which originally compose the skeletal plate, are afterwards converted for the most part into cartilaginous cells, and these secrete a firm and elastic intercellular substance between them, and form cartilaginous tissue.
We must certainly regard it as a comparative-anatomical and ontogenetic fact of the greatest significance that in all the Craniotes, from the lowest Cyclostomes and fishes up to the apes and man, the brain develops in just the same way in the embryo. The first rudiment of it is always a simple vesicular enlargement of the fore end of the medullary tube.
If we compare the fully-developed arterial system of the various classes of Craniotes, it shows a good deal of variety, yet it always proceeds from the same fundamental type. At first there is only a single pair of arches, which lie on the inner surface of the first pair of gill-arches. Finally, we get a fourth, fifth, and sixth pair.
Opposed to the Acrania is the second division of the vertebrates, which comprises all the other members of the stem, from the fishes up to man. Hence they are called the Craniota. These Craniotes are, however, without a skull in their earlier period.
Just as to-day the intricate structure of the brain proceeds step by step from the same rudiment in every human individual the same five cerebral vesicles as in all the other Craniotes; so the human soul has been gradually developed in the course of millions of years from a long series of craniote-souls.
Through the cochlear nerves we learn the height and timbre, through the vestibular nerves the intensity, of tones. The first structure of this highly elaborate organ is very simple in the embryo of man and all the other Craniotes; it is a pit-like depression in the skin. This sinks into a sort of pit, and severs from the epidermis, just as the lens of the eye does.
In this way the central marrow of the Vertebrates divides clearly into its two chief sections, brain and spinal cord. But in these it soon passes away, the one vesicle being divided into several successive parts by transverse constrictions. In all the Craniotes, from the Cyclostoma up to man, the same parts develop from these five original cerebral vesicles, though in very different ways.
As we find this highly characteristic differentiation of the gut into two different sections in all the Vertebrates and all the Tunicates, we may conclude that it was also found in their common ancestors, the Prochordonia especially as even the Enteropneusts have it. But the number presently increases in the former. In the Craniotes, however, it decreases still further.
The simple, spindle-shaped heart-tube, found in the same form in the embryo of all the Craniotes, is divided into two sections or chambers in the Cyclostomes, and these are separated by a pair of valves. The hind section, the auricle, receives the venous blood from the body and passes it on to the anterior section, the ventricle.
We should be quite unable to explain these if comparative anatomy and ontogeny did not come to our assistance. The vascular system in man and all the Craniotes is an elaborate apparatus of cavities filled with juices or cell-containing fluids. With the latter are connected the large cavities of the body, especially the body-cavity, or coeloma.
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