The Origin of Vertebrates

by Walter Holbrook Gaskell

The interpretation of this stage is that in the invertebrate ancestor the nerve-masses were situated laterally and ventrally to the epithelial tube, and were connected together by commissures on the ventral side of the tube (Fig. [21], A (1)); in other words, the chain of ventral ganglia and their transverse commissures lying just ventrally to the intestine, which are so characteristic of the arthropod nervous system, is represented at this stage.

Fig. 21.—A, Method of Formation of the Vertebrate Spinal Cord from the Ventral Chain of Ganglia and the Intestine of an Arthropod, represented in 1; B, Method of Formation of the Vertebrate Medulla Oblongata from the Infra-œsophageal Ganglia and the Cephalic Stomach of an Arthropod.

Subsequently, by the growth dorsalwards of nervous material to form the posterior columns, the original epithelial tube is compressed dorsally and laterally to such an extent that those parts lose all signs of lumen, the one becoming the posterior fissure and the others the substantia gelatinosa Rolandi on each side. The original tube is thus reduced to a small canal formed by its ventral portion only (Fig. [21], A (3)). In this way the spinal cord is formed, and the walls of the original epithelial tube are finally visible only as the lining of the central canal (Fig. [21], A (4)).

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; 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 Ammocœtes 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.

The infra-infundibular nervous mass is naturally confined to the ventral side of the epithelial tube, because it represents the infra-œsophageal ganglia, situated as they are on the ventral side of the cephalic stomach, and, owing to the size of the stomach, they could not enclose it by dorsal growth, as they do in the case of the formation of the spinal cord (Fig. [21], B (1)). Still these nervous masses have grown dorsalwards, have commenced to involve the walls of the cephalic stomach even in the lowest vertebrate, as is seen in Ammocœtes, in which animal a ventral portion of the epithelial bag has been evidently compressed and its lumen finally obliterated by the growth of the nerve-masses on each side of it. Throughout the whole vertebrate kingdom this obliterated portion still leaves its mark as the raphé or seam, which is so characteristic of the infra-infundibular portion of the brain.

Fig. 22.—Horizontal Section through the Brain of Ammocœtes.