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We owe to it the most important data that we proceed on in reconstructing the gradual historical development of the whole stem. All the Craniota descend from a common stem-form, and this was substantially identical in structure with the Amphioxus.
But before we consider the development of the complicated structure of the brain from this simple series of vesicles, let us glance for a moment at the lower animals, which have no brain. Even in the skull-less vertebrate, the Amphioxus, we find no independent brain, as we have seen.
Then the candidate was badgered about the pterodactyl, and concerning the difference in anatomy between a bat and a bird, and about the lamprey, and the cartilaginous fishes, and the amphioxus. All these questions he answered more or less to the satisfaction of the examiners generally less.
We will now compare the embryonic development of the two animals, and find to our great astonishment that the same embryonic form develops from the ovum of the Amphioxus as from that of the Ascidia a typical chordula.
This is the stem of the Tunicates. One member of this group, the sea-squirt, very closely approaches the lowest vertebrate, the Amphioxus, in its essential internal structure and embryonic development.
Our comparative investigation of the anatomy and ontogeny of the Amphioxus and Ascidia has given us invaluable assistance.
Professor Haeckel proposes to break up the vertebrate sub-kingdom, first, into the two provinces of Leptocardia and Pachycardia; Amphioxus being in the former, and all other vertebrates in the latter division.
It is evident that the Amphioxus from the vertebrate side and the Ascidia from the invertebrate form the bridge by which we can span the deep gulf that separates the two great divisions of the animal kingdom.
As the same section of the ventral vessel, which also forms the heart in the Craniotes, has developed in the Ascidia into a simple tubular heart, we may regard the absence of this in the Amphioxus as a result of degeneration, a return in this case to the earlier form of the vascular system, as we find it in many of the worms.
However, in the lamprey the spinal cord swells in front into a simple pear-shaped cerebral vesicle, and at each side of it there are a very simple eye and a rudimentary auditory vesicle. The nose is a single pit, as in the Amphioxus. The two sections of the gut are also just the same and very rudimentary in the lamprey.
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