The Origin of Vertebrates

by Walter Holbrook Gaskell

The evidence of embryology points directly to the greater phylogenetic antiquity of the cranial region, for we see how, quite early in the development, the head is folded off, and the organs in that region thereby completed at a time when the spinal region is only at an early stage of development. We see how the first of the trunk somites is formed just posteriorly to the head region, and then more and more somites are formed by the addition of fresh segments posteriorly to the one first formed. We see how, in Ammocœtes, the first formed parts of the skeleton are the branchial bars and the basi-cranial system, while the rudiments of the vertebræ do not appear until the Petromyzon stage. We see how, with the elongation of the animal by the later addition of more and more spinal segments, organs, such as the heart, which were originally in the head, travel down, and the vagus and lateral-line nerves reach their ultimate destination. Again, we see that, whereas the cranial nerves, viz. the ocular motor, the trigeminal, facial, auditory, glossopharyngeal, and vagus nerves, are wonderfully fixed and constant in all vertebrates, the only shifting being in the spino-occipital region, in fact, at the junction of the cranial and spinal region, the spinal nerves, on the other hand, are not only remarkably variable in number in different groups of animals, but that even in the same animal great variations are found, especially in the manner of formation of the limb-plexuses. Such marked meristic variation in the spinal nerves, in contrast to the fixed character of the cranial nerves, certainly points to a more recent formation of the former nerves.

Also the observations of Assheton on the primitive streak of the rabbit, and on the growth in length of the frog embryo, have led him to the conclusion that, as in the rabbit so in the frog, there is evidence to show that the embryo is derived from two definite centres of growth: the first, phylogenetically the oldest, being a protoplasmic activity, which gives rise to the anterior end of the embryo; the second, one which gives rise to the growth in length of the embryo. This secondary area of proliferation coincides with the area of the primitive streak, and he has shown, in a subsequent paper, by means of the insertion of sable hairs into the unincubated blastoderm of the chick, that a hair inserted into the centre of the blastoderm appears at the anterior end of the primitive streak, and subsequently is found at the level of the most anterior pair of somites.

He then goes on to say—

"From these specimens it seems clear that all those parts in front of the first pair of mesoblastic somites—that is to say, the heart, the brain and medulla oblongata, the olfactory, optic, auditory organs and foregut—are developed from that portion of the unincubated blastoderm which lies anterior to the centre of the blastoderm, and that all the rest of the embryo is formed by the activity of the primitive streak area."

In other words, the secondary area of growth, i.e. the primitive streak area, includes the whole of the spinal cord region, while the older primary centre of growth is coincident with the cranial region.

In searching, then, for the origin of the segmental nerves, we must consider the type on which the cranial nerves are arranged rather than that of the spinal nerves.

The first striking fact occurs at the spino-occipital region, where the spinal cord merges into the medulla oblongata, for here in the cervical region we find each spinal segment gives origin to three distinct roots, not two—a dorsal root, a ventral root, and a lateral root. This third root gives origin to the spinal accessory nerve, and in the region of the medulla oblongata these lateral roots merge directly into the roots of the vagus nerve; more anteriorly the same system continues as the roots of the glossopharyngeal nerve, as the roots of the facial nerve, and as a portion, especially the motor portion, of the trigeminal nerve. Now, all these nerves belong to a well-defined system of nerves, as Charles Bell[^[1]] pointed out in 1830, a system of nerves concerned with respiration and allied mechanisms, such as laughing, sneezing, mastication, deglutition, etc., nerves innervating a set of muscles of very different kind from the ordinary body-muscles concerned with locomotion and equilibration. Also the centres from which these motor nerves arise are well defined, and form cell-masses in the central nervous system, quite separate from those which give origin to somatic muscles.

This original idea of Charles Bell, after having been ignored for so long a time, is now seen to be a very right one, and it is an extraordinary thing that his enunciation of the dual nature of the spinal roots, which was, to his mind, of subordinate importance, should so entirely have overshadowed his suggestion, that in addition to the dorsal and ventral roots, a lateral system of nerves existed, which were not exclusively sensory or exclusively motor, but formed a separate system of respiratory nerves.

Further, anatomists divide the striated muscles of the body into two great natural groups, characterized by a difference of origin and largely by a difference of appearance. The one set is concerned with the movements of internal organs, and is called visceral, the other is derived from the longitudinal sheet of musculature which forms the myotomes of the fish, and has been called parietal or somatic. The motor nerves of these two sets of muscles correspond with the lateral or respiratory and ventral roots respectively.

Finally, it has been shown that the segments of which a vertebrate is composed are recognizable in the embryo by the segmented manner in which the musculature is laid down, and van Wijhe has shown that in the cranial region two sets of muscles are laid down segmentally, thus forming a dorsal and ventral series of commencing muscular segments. Of these the anterior segments of the dorsal series give origin to the striated muscles of the eye which are innervated by the IIIrd (oculomotor), IVth (trochlearis), and VIth (abducens) nerves, while the posterior segments give origin to the muscles from the cranium to the shoulder-girdle, innervated by the XIIth (hypoglossal) nerve. The ventral series of segments give origin to the musculature supplied by the trigeminal, facial, glossopharyngeal, and vagus nerves.