The most remarkable evidence of all has been afforded by Miss Alcock's experiments. She examined the different tissues of Ammocœtes for the express purpose of finding out their power of digesting fibrin, with the result that the most active cells were those of the liver. Next in activity came the extract of the lining cells of the respiratory chamber and of the skin. The intestine itself when freed from the liver-secretion had very little digestive power; extracts of muscle, nervous system, and thyroid gland had no power whatever, but the extract of the skin-cells possessed a powerful digesting action.
Furthermore, it is not necessary to make an extract of the skin in order to obtain this digestive fluid, for under the influence of chloroform the skin of Ammocœtes secretes copiously, and this fluid thus secreted was found to possess strong digestive powers. So, also, Miss Alcock has demonstrated the power of digesting fibrin in a similar secretion of the epithelial cells lining the carapace of the crayfish. In both cases a very plausible reason for the presence of a digestive ferment in a skin-secretion is found in the necessity of preventing the growth of parasites, fungoid, or otherwise, especially in those parts where the animal cannot keep itself clean by 'preening.' Thus in a crayfish, in which the œsophageal commissures had been cut, fungus was found to grow on the ventral side, but not on the dorsal carapace. The animal was accustomed to keep its ventral surface clean by preening; owing to the paralysis it could not do so, and consequently the fungus grew there. In the lamprey I found that wherever there was a removal of the surface-epithelium, from whatever cause, that spot was immediately covered with a fungoid growth, although in the intact lamprey the skin was invariably smooth and clean.
I imagine, then, that this digestive power of the skin arose as a protective mechanism against parasitic attacks; it is self-evident how a tube formed of such material must ab initio act as a digestive tube.
In yet another respect this skin secretion of Ammocœtes is most instructive. The surface of Ammocœtes is absolutely smooth, no scales of any kind exist; this smoothness is due to the presence of a very well-defined cuticular layer secreted by the underlying epithelial cells. This cuticle is very much thicker than is usually found in vertebrates, and, strangely enough, has been thought to contain chitin. Whether it really contains chitin or not I am unable to say, but it certainly resembles a chitinous layer in one respect; it is perforated by innumerable very fine tubes or canaliculi, along which, by appropriate staining, it is easy to see the secretion of the underlying cell pass to the exterior (Fig. [140]). This marked digestive power of the skin of Ammocœtes, together with the easy passage of the secretion through the thin cuticular layer, renders it almost certain that a tube formed from the deep ventral groove of the trilobite would, from the very first, act as a digestive as well as an absorbent tube; in other words, the notochord as soon as formed was able to act as an accessory digestive tube.
This suggested origin of the notochord from a groove along the mid-ventral surface of the body not only indicates a starting-point from a markedly segmented portion of the body, but also points to its formation at a stage previous to the formation of the operculum by the fusion of the two foremost mesosomatic appendages—indicates therefore its formation at a stage more nearly allied to the trilobite than to the sea-scorpion. The chance of ever finding any direct evidence of such a chordate trilobite stage appears to me exceedingly improbable, and I greatly fear that this conception of the mode of formation of the notochord can never be put to direct proof, but must always remain guesswork.
On the other hand, evidence of a kind in favour of its origin from a segmented part of the body does exist, and that evidence has this special value, that it is found only in that most primitive animal, Amphioxus.
This evidence is as follows:—
At fairly regular intervals, the sheath of the notochord is interrupted on each side of the mid-dorsal line by a series of holes, which penetrate the whole thickness of the sheath. This dorsal part is pressed closely against the spinal cord, and through these holes fibres appear to pass from the spinal cord to the interior of the notochord. So greatly do these fibres present the appearance of ventral roots to the notochord, that Miss Platt looks upon them as paired motor roots to the notochord, or at all events as once having been such motor roots. Lwoff and Rolph both describe a direct communication between the spinal cord and the notochord by means of fibres passing through these holes, without however looking upon this connection as a nervous one. Joseph alone asserts that no absolute connection exists, for the internal elastic layer of the notochord, according to him, is not interrupted at these holes, and forms, therefore, a barrier between the fibres from the spinal cord and those from the interior of the notochord. Still, whatever is the ultimate verdict as to these fibres, the suggestive fact remains of the spaces in the notochordal sheath and of the corresponding projecting root-like fibres from the spinal cord. The whole appearance gives the impression of some former connection, or rather series of connections, between the spinal cord and the notochord, such as would have occurred if nerves had once passed into the notochord. On the other hand, such nerves were not arranged segmentally with the myotomes, for, according to Joseph, in the middle of the animal ten to twelve such holes occur in one body-segment. In Apus the appendages are more numerous than the body-segments, so that it is not necessary for a segmental arrangement to coincide with that of the body-segments.
The Origin of the Alimentary Canal.
In close connection with the notochord is the alimentary canal. Any explanation of the one must be of assistance in explaining the other.