the embryonic brain. 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. The whole central marrow is merely a simple cylindrical cord which runs the length of the body, and ends equally simply at both extremities--a plain medullary tube. All that we can discover is a small vesicular bulb at the foremost part of the tube, a degenerate rudiment of a primitive brain. We meet the same simple medullary tube in the first structure of the ascidia larva, in the same characteristic position, above the chorda. On closer examination we find here also a small vesicular swelling at the fore end of the tube, the first trace of a differentiation of it into brain and spinal cord. It is probable that this differentiation was more advanced in the extinct Provertebrates, and the brain-bulb more pronounced (Figures 1.98 to 1.102). The brain is phylogenetically older than the spinal cord, as the trunk was not developed until after the head. If we consider the undeniable affinity of the Ascidiae to the Vermalia, and remember that we can trace all the Chordonia to lower Vermalia, it seems probable that the simple central marrow of the former is equivalent to the simple nervous ganglion, which lies above the gullet in the lower worms, and has long been known as the "upper pharyngeal ganglion" (ganglion pharyngeum superius); it would be better to call it the primitive or vertical brain (acroganglion). Probably this upper pharyngeal ganglion of the lower worms is the structure from which the complex central marrow of the higher animals has been evolved. The medullary tube of the Chordonia has been formed by the lengthening of the vertical brain on the dorsal side. In all the other animals the central nervous system has been developed in a totally different way from the upper pharyngeal ganglion; in the Articulates, especially, a pharyngeal ring, with ventral marrow, has been added. The Molluscs also have a pharyngeal ring, but it is not found in the Vertebrates. In these the central marrow has been prolonged down the dorsal side; in the Articulates down the ventral side. This fact proves of itself that there is no direct relationship between the Vertebrates and the Articulates. The unfortunate attempts to derive the dorsal marrow of the former from the ventral marrow of the latter have totally failed (cf. Chapter 2.20). (FIGURE 2.293. The human brain, seen from the left. (From H. Meyer.) The furrows of the cerebrum are indicated by thick, and those of the cerebellum by finer lines. Under the latter we can see the medulla oblongata. f1 to f2 frontal convolutions, C central convolutions, S fissure of Sylvius, T temporal furrow, Pa parietal lobes, An angular gyrus, Po parieto-occipital fissure.)