The Origin of Vertebrates

by Walter Holbrook Gaskell

According to the prevalent embryological teaching, the body is formed of three layers, epiblast, hypoblast, and mesoblast, and the gastræa theory of the origin of all Metazoa implies of necessity that the formation of every individual commences with the formation of the gut. For this reason the alimentary canal must in every case be regarded as the earliest formed organ, however late in the development it may attain its finished appearance. Hence the notochord is spoken of as developed from the mid-dorsal wall of the alimentary canal. It is possible to look at the question the other way round, and suppose that the organ whose development is finished first is older than the one still in process of making. In this case it would be more right to say a ventral extension of the tissue, which gives rise to the notochord, takes place and forms the alimentary canal. It is, to my mind, perfectly possible, and indeed probable, that the formation of the vertebrate alimentary canal was a repetition of the same process which had already led to the formation of the notochordal tube. The formation of the anterior part of the alimentary canal in Ammocœtes at the time of transformation strongly suggests the marked similarity of the two processes.

Of all the startling surprises which occur at transformation, this formation of a new anterior gut is the most startling. From the oral chamber of Petromyzon two tubes start: the one leads into the gill-chambers, is known as the bronchus, and is entirely concerned with respiration; the other leads without a break from the mouth to the anus, has no connection with respiration, and is the alimentary canal of the animal. Any one looking at Petromyzon would say that its alimentary canal was absolutely non-respiratory in character. Before transformation, this kind of alimentary canal commences at the end of the respiratory chamber; from here to the anus it is of the same character as in Petromyzon, but in Ammocœtes the non-respiratory anterior part simply does not exist: the whole anterior chamber is both respiratory and affords passage to food. This part of the alimentary canal of the adult is formed anew. We see, then, here the formation of a part of the alimentary canal taking place, not in an embryo full of yolk, but in a free-living, independent, grown-up larval form in which all yolk has long since disappeared: a condition absolutely unique in the vertebrate kingdom, but one which more than any other may be expected to give a clue to the method of formation of a vertebrate gut.

The formation of this new gut can be easily followed at transformation, and was originally described by Schneider. His statement has been confirmed by Nestler, and its absolute truth has been demonstrated to me again and again by Miss Alcock, in her specimens illustrative of the transformation process. First, in the mid-dorsal line of the respiratory chamber a distinct groove is formed, the edges of which come together and form a solid rod. This solid rod blocks the opening of the respiratory chamber into the mid-gut, so that during this period of the transformation no food can pass out of the pharyngeal chamber. A lumen then begins to appear in this solid rod at the posterior end, which steadily advances mouthwards until it opens into the oral chamber and thus forms an open tube connecting the mouth with the gut.

Here, then, is the foundation of a new gut on very similar lines to that of the notochord, by the conversion of a groove into a tube. Still more suggestive is it to find that the tube so formed has no appearance whatever of segmentation; it is as unsegmented as the rest of the gut, although, as is seen in Fig. [62], the dorsal wall of the respiratory chamber from which it arose is as markedly segmented as any part of the animal. Here under our very eyes, in the course of a few days or weeks, an object-lesson in the process of the manufacture of an alimentary canal is carried out and completed, and the teaching of that lesson is that a gut-tube may be formed in the same way as the notochordal tube, by the conversion of a grooved surface into a canal, and that gut-tube so formed, like the notochord, loses all sign of segmentation, even although the original grooved surface was markedly segmented.

The suggestion then is, that the new gut may have been formed by a repetition of the same process which had already given origin to the notochord.

Such a method of formation is not, in my opinion, opposed to the evidence given by embryology, but in accordance with it; the discussion of this point will come best in the next chapter, which treats of the embryological evidence as a whole, and will therefore be left till then.

The Evidence given by the Innervation of the Vertebrate Alimentary Canal.

Throughout this investigation the one fixed landmark to which all other comparisons must be referred, is the central nervous system, and the innervation of every organ has given the clue to the meaning of that organ. So also it must be with the new alimentary canal; by its innervation we ought to obtain some insight into the manner of its origination. In any organ the nerves which are specially of value in determining its innervation, are of necessity the efferent or motor nerves, for the limits of their distribution in the organ are much more easily determined than those of the afferent or sensory nerves. The question therefore of primary importance in endeavouring to determine the nature of the origin of the alimentary canal from its innervation is the determination of the efferent supply to the musculature of its walls.

Already in previous chapters a commencement has been made in this direction; thus the musculature of the oral chamber has been derived directly from the musculature of the prosomatic appendages; the muscles which move the eyes from the prosomatic and mesosomatic dorso-ventral somatic muscles; the longitudinal body-muscles from the dorsal longitudinal somatic muscles of the arthropod; the muscles of respiration from the dorso-ventral muscles of the mesosomatic appendages.

In all these cases we have been dealing with striated musculature and consequently with only the motor nerves of the muscle; but the gut posterior to the pharyngeal or respiratory chamber contains unstriped instead of striped muscle, and is innervated by two sets of nerves, those which cause contraction and are motor, and those which cause relaxation and are inhibitory. It is by no means certain that these two sets of nerves possess equal value from a morphological point of view. The meaning of an inhibitory nerve is at present difficult to understand, and in this instance, is rendered still more doubtful owing to the presence of Auerbach's plexus along the whole length of the intestine—an elaborate system of nerve-cells and nerve-fibres situated between the layers of longitudinal and circular muscles surrounding the gut-walls, which has been shown by the recent experiments of Magnus, to constitute a special enteric nervous system.