Page:Popular Science Monthly Volume 82.djvu/437

Rh is there known as the infundibulum and the saccus vasculosus; it was completely shut off from the outside world by the closing of the brain tube during the process of embryonic growth. (4) The present mouth of the vertebrates is a new one; it arose through the transformation of the so-called "dorsal organ," or cephalic navel, an organ of obscure function, but one that is present in all arthropods near the place where the new mouth of the vertebrates ultimately appears. In the arthropods it may serve as an organ of attachment, and at certain stages of development affords a temporary passageway into the alimentary canal. In other words, the arachnid brain could not continue to grow in volume, and in the particular way in which it had been growing, without closing up the old mouth, and without forcing the jaws farther and farther apart, until they reached the opposite side of the head. Here they converged toward the cephalic navel, which then became a permanent opening and was utilized as a new mouth to take the place of the old one that was being closed up.

Again, in still other words, we are here dealing with a series of interlocking, internal organic adjustments that ultimately reached a condition of unstable equilibrium. A radical and comparatively rapid readjustment then took place, which led to a new condition of great stability. This situation constituted a great crisis in the evolution of this phylum, perhaps the most momentous crisis in the history of organic evolution. But these revolutionary events were brought about in an intelligible way by the cumulative action of long-established methods of growth, which can be traced through the arachnid stock up to the point where the ensuing events appear inevitable. The actual consummation of them marks the transition from the vertebrates to the invertebrates, and the beginning of a new class of animals. From the nature of the case, the transition must have taken place somewhat rapidly, and it probably occurred at some time during, or before, the Silurian period. The paired jaws of the adult ostracoderms are intelligible only on the assumption that they represent one of the early phylogenetic stages of this process.

The general way in which this metamorphosis took place is still recorded in the embryonic history of the vertebrates to the present day, for we can readily observe, in many vertebrate embryos, the shutting up of the old mouth within the brain chamber, the transfer of at least three pairs of jaws to the opposite side of the head, and their union around the new mouth.

In the embryo of the frog, for example, three pairs of rudimentary