When we pass to the brain-region, to the anterior dilated portion of the tube, embryology tells a different story. Here, as in the spinal cord, the nervous masses are grouped at first laterally and ventrally to the epithelial tube, as is seen in Fig. 21, B (2), but owing to the large size of its lumen here, the nervous material is not able to enclose it completely, as in the case of the spinal cord; { 45}consequently there is no posterior fissure formed; but, on the contrary, the dorsal roof, not enclosed by the nerve-masses, remains epithelial, and so forms the membranous roof of the fourth ventricle and of the other ventricles of the brain (Fig. 21, B (3)). In the higher animals, owing to the development of the cerebrum and cerebellum, this membranous roof becomes pushed into the larger brain cavity, and thus forms the choroid plexuses of the third and lateral ventricles. In the lower vertebrates, as in Ammocoetes and the Dipnoi, it still remains as a dorsal epithelial roof and forms a most striking characteristic of such brains.
In this part of the nervous system, then, the nervous material is all grouped in its original position on the ventral side of the tube; and yet it is the same nervous material as that of the spinal cord, all the elements are there, giving origin here to the segmental cranial nerves just as lower down they give rise to the segmental spinal nerves, connecting together the separate segments each with the other and all with the higher brain-centres--the supra-infundibular centres--just as they do in the spinal region.
Why should there be this striking difference between the formation of the infra-infundibular region of the brain and that of the spinal cord? Do the advocates of the origin of vertebrates from Balanoglossus give the slightest reason for it? They claim that their view also provides a tubular nervous system for the vertebrate, but give not the slightest sign or indication as to why the nervous material should be grouped entirely on the ventral side of an epithelial tube in the infra-infundibular region and yet surround it in the spinal cord region. And the explanation is so natural, so simple: embryology does its very best to tell us the past history of the race, if only we look at it the right way.