Art. XXIV.—The Life-history of the Tuatara (Sphenodon punctatum).
[Read before the Philosophical Institute of Canterbury, 22nd February, 1899]
The results of my researches on the development of the Tuatara having been sent to England for publication,* it has been thought desirable to print a short account of the life-history of this remarkable animal in the “Transactions of the New Zealand Institute,” in the hope that it may prove of interest to the members.
In 1896 I succeeded in making arrangements with Mr. P. Henaghan, then principal keeper on Stephens Island, in Cook Strait, for obtaining a supply of eggs, and, thanks to the untiring zeal displayed by my correspondent in the interests of science, these arrangements proved eminently successful. Mr. Henaghan also furnished me with very valuable information
[Footnote] * “Summary of the Principal Results obtained in a Study of the Tuatara (Sphenodon punctatum).”—(“Proceedings of the Royal Society of London,” vol. 63, 1898.)
[Footnote] “Outlines of the Development of the Tuatara (Sphenodon (Hatteria) punctatum).”—(“Quarterly Journal of Microscopical Science”: In the Press.)
[Footnote] “On the Development of the Parietal Eye and Adjacent Organs in Sphenodon (Hatteria).”—(“Quarterly Journal of Microscopical Science”: In the Press.)
concerning the breeding habits of the Tuatara, which were previously almost entirely unknown.* The eggs were sent to my laboratory packed in damp earth or sand, in tin cans, and when sent only a few at a time were found to travel very well. Indeed, later on, after the discovery of the hibernation of the embryo within the egg, my wife even took eggs to England with the living embryos in them, and they arrived safely at their destination. In the early stages of development, however, it is not very easy to prevent the eggs from either shrivelling up or going mouldy. Owing to these satisfactory arrangements it became unnecessary for me to visit Stephens Island personally, as I had thought of doing, and I was able to prepare and examine the embryos with all the conveniences afforded by my laboratory at the Canterbury College.
It appears that the failure of earlier collectors to obtain the eggs of the Tuatara was due to absence of information as to the breeding habits. It is well known that the adult animals live in holes, which are frequented also by sea-birds, and, indeed, the young of the birds serve them for food. The Tuataras, however, do not, as a rule, lay their own eggs in these holes, as was naturally enough supposed. On the other hand, Mr. Henaghan found that they make special holes for the reception of their eggs. In each of these “nests” from ten to fifteen eggs are laid, apparently by a single female, and the hole is then carefully filled in and concealed with grass or leaves.
The eggs are laid on Stephens Island during the month of November, and it is a very remarkable fact that they do not hatch until about midsummer of the year following. The earlier stages of the development are passed through much more rapidly than the later ones, and about the month of March, having already reached a very advanced stage, the development of the embryo is almost suspended for the winter months, being resumed again in the following spring. This hibernation of the embryo within the egg has been observed in only one other vertebrate animal, and that is the common European tortoise.† Certain other developmental features also indicate a close relationship between the Tuatara and the Chelonians (turtles and tortoises).
The eggs, when newly laid, are usually rather more than an inch long, and of broadly oval shape. They are almost entirely filled with yellow yolk, and contain very little albumen, or “white.” As the time of hatching approaches the
[Footnote] * See, however, Professor Thomas's paper, “Preliminary Note on the Development of the Tuatara (Sphenodon punctatum).”—(“Proceedings of the Royal Society of London,” vol. 48; and “New Zealand Journal of Science,” vol. 1, 1891.)
[Footnote] † Emis orbicularis. Vide Boulenger (“Nature,” 27th October, 1898).
eggs swell considerably, doubtless by absorption of moisture through the leathery egg-shell. I attribute this absorption of moisture to the hygroscopic action of a semi-gelatinous fluid which is secreted in large quantity in the cavity of the enormously developed allantois. Finally, a very high state of tension is reached, when a very small incision through the egg-shell is sufficient to cause it to split open with almost explosive violence, as I twice experimentally determined.
In the later stages of development a patch of horny epidermis on the snout of the young animal forma a sharp cutting instrument, which is doubtless used for the purpose of making the necessary incision in the egg-shell. A similar structure may be observed in the chick and in numerous other vertebrate embryos.
The embryos obtained form a very perfect series, and have been classified and described under sixteen stages, distinguished by the letters of the alphabet from C to S. For a detailed description of these stages, with the necessary illustrations, I must refer the reader to my monograph on the development. It will suffice here to give a very brief sketch of the life-history.
The earlier stages closely resemble those of the tortoise, especially as regards the development of the fœtal membrane known as the amnion. This structure, as in other vertebrates, surrounds the embryo in the form of a bag or sac as it lies on the surface of the yolk. In the Tuatara this sac is continued behind the embryo for some distance, in the form of a long narrow canal or tunnel, open posteriorly. Such a structure as this “posterior amniotic canal” was entirely unknown in any animal until a few years ago, when it was discovered by the Japanese embryologist, Professor Mitsukuri, in the embryos of a tortoise. Since the publication of the summary of my observations in June last. Professor Mitsukuri has kindly sent me a copy of his work, and it is really extraordinary to see how closely his drawings of the tortoise embryo agree with mine of the Tuatara. Another feature in which the young embryos of the Tuatara agree closely with those of the tortoise is the curious manner in which the head dips down into the yolk, enveloped in a peculiar membrane known as the “pro-amnion.”
In general features, however, the earlier stages of the development closely resemble those of other vertebrates with heavily yolked eggs. The formation of the alimentary canal, the four pairs of gill-slits in the neck, the central nervous system, the eyes, ears, and nostrils, the notochord, the limbs, & c., takes place in the manner already well known in other types.
At stage R, however, during which the hibernation occurs,
the embryo exhibits characters of special interest. It is now far advanced in development, though there is still a very large quantity of unused yolk in the yolk-sac. The limbs are well formed, the tail is long, and the head large and very similar to that of a tortoise, a resemblance which is to a large extent retained even in the adult. On the snout is the sharp-pointed shell-cutter; and the body is marked with longitudinal and transverse bands or stripes of alternate grey and white, a pattern which is completely lost in the adult animal, which is characteristically spotted. Thus the Tuatara conforms to what appears to be a general law of coloration amongst the higher vertebrates, for it has been observed in many different types—e.g., the emu—that the young animal is striped even when the adult is not striped; and there is good, ground for believing that striping was the first kind of pattern to make its appearance in the ancestors of existing vertebrates.
Perhaps the most remarkable feature of stage R, however, is the plugging-up of the nostrils by a dense growth of cellular tissue, and it is a truly remarkable fact that the only other animal in which this embryonic character has been observed is the Kiwi (Apteryx), in which it was described some years since by the late Professor Parker. Why should two animals so widely separated zoologically as the Tuatara and the Kiwi, and both confined to New Zealand, exhibit this extraordinary feature of development? Has the plugging-up of the nostrils in the Tuatara any relation to the hibernation of the embryo, and, if so, why does the Kiwi exhibit the same character? These questions I fear it is impossible to answer in the present state of our knowledge.
At stage S the embryo has acquired nearly all the characters of the adult: the yolk is all absorbed, and the young animal hatches. One fact only needs to be mentioned here about this stage, and that concerns the teeth. In the adult Tuatara, as is well known, there are two very large cutting-teeth in front of each jaw, upper and lower. At stage S each of these four teeth is represented by three separate conical cusps, which evidently grow together with advancing age to form the very characteristic front teeth of the adult.
As regards the details of the development of the special organs much still remains to be done, and in this work I have been very fortunate in securing the co-operation of eminent specialists in England. Professor G. B. Howes, LL.D., F.R.S., has most kindly consented to undertake the investigation of the development of the skeleton, and has already commenced the work, with the assistance of one of his students. Happily I received a further supply of eggs from Mr. Henaghan in December last, which have afforded much valuable material for further investigation. Two other English specialists have also
kindly offered to investigate the detailed development of the brain and the excretory organs, but as the arrangements are not yet complete I do not feel at liberty to publish their names.
The development of that remarkable organ, the parietal or so-called “pineal” eye, might naturally have been expected to prove of exceptional interest, and these expectations have not been disappointed. This part of the subject I have myself worked out in detail, and my results have been embodied in a separate memoir. It will be remembered that in the adult Tuatara the parietal eye, although quite invisible externally, exhibits a higher degree of perfection in structure than in perhaps any other known type. This structure was first described for Sphenodon by Professor Baldwin Spencer. During recent years a large amount of literature has been published by various authors on the structure and development of the parietal eye in divers types of lizards, and on the corresponding structures met with in lower vertebrate types (fishes). As a result of these researches, taken in conjunction with my own observations on the development of the parietal eye in the Tuatara, we may consider ourselves justified in concluding that the ancestors of existing vertebrates possessed, in addition to the ordinary paired eyes, a pair of parietal eyes placed side by side on top of the head, and originating as outgrowths of the brain, and perhaps serially homologous with the ordinary paired eyes.
In existing sharks (Selachians) Locy has shown that the two parietal optic vesicles unite together in the middle line to form the so-called “epiphysis.”
In bony fishes (Teleosts and Amia) Hill has shown that there is also a pair of outgrowths arising in a similar way from the brain, but with more or less displacement. In these fishes, however, the right vesicle alone gives rise to the “epiphysis” of the adult, while the left one separates completely from the brain, and undergoes degeneration.
In lampreys (Cyclostomes) there is again a similar pair of outgrowths, which suffer displacement in such a manner that the right vesicle comes to overlie the left. Here the right vesicle forms a fairly well organized parietal eye, and the left one the so-called “parapineal organ,” and the two together, with the nerve of the parietal eye, form what is usually known as the “epiphysis” in this group.
At a very early stage in the development of the Tuatara the left parietal eye (optic vesicle) appears as an outgrowth from the fore-brain, slightly to the left of the middle line. The right parietal optic vesicle appears slightly later, and never attains anything like the same degree of organization as the left one, although exhibiting essentially the same struc-
ure. It is displaced so as to lie behind and beneath the left parietal eye, and the latter gradually shifts into the middle line. Thus the left parietal eye appears finally as a median unpaired organ, and the right parietal eye as a median elongated vesicle behind and beneath it. This very degenerate right parietal eye has been termed the “parietal stalk,” while the left parietal eye was formerly assumed to have been developed by a kind of nipping-off of the end of the stalk, which in turn was more or less closely identified with the “epiphysis” or “pineal gland.”
In the Tuatara, as already stated, the left parietal eye is very highly developed. It originates as a simple hollow outgrowth of the fore-brain. This soon separates from the brain, and forms a closed sac, lying beneath the integument of the top of the head. The upper part of the wall of this sac, the skin, thickens, and forms a very well developed biconvex lens, composed of elongated cells. The lower part of the wall forms the retina, which early becomes divided into two primary layers. In connection with the outer layer of the retina a special optic nerve is developed, and in the inner layer pigment is deposited. The large cavity of the eye, between the lens and the retina, becomes filled, with a coagulable humour.
In lizards (Lacertilia) the development of the parietal eye appears to follow much the same course as in the Tuatara, although perhaps its paired origin has not been so clearly recognised, owing to the greater extent to which the parts concerned have undergone degeneration.
In the Tuatara, and in lizards, and perhaps in all the higher vertebrates, the so-called “epiphysis” or “pineal gland” is a composite structure formed by various outgrowths of the brain, of which the parietal eyes, or their degenerate representatives, form only a very small part.
Since the Tuatara is recognised as being the oldest surviving type of terrestrial vertebrate, belonging to a family—the Rhynchocephalia—which dates back to the Palæozoic (Permian) epoch, and which is now on the verge of extinction, * we might expect that its development would exhibit primitive features, and throw light upon the ancestral history of the higher vertebrates in general. To a certain extent, no doubt, this is the case; but we may expect more light to be thrown upon this subject by the comparison of the different stages in the development of the skeleton with the fossil remains of extinct vertebrate types, and until the development of the skeleton is worked out it would be rash to make any wide generalisations.
[Footnote] * The Tuatara is the only surviving member of this family, and it is now confined to certain small islands off the coast of New Zealand.
In the meantime, I may repeat that as regards the parietal eye the early stages of development agree exactly with those of bony fishes while, as regards the fœtal membranes, there is an equally striking agreement with the Chelonians (turtles and tortoises), to which group some of our most eminent zoologists—e.g., Boulenger—have long supposed the Tuatara to be closely related. The views of these authorities have received startling confirmation from the study of the development of the Tuatara.
Considering the evidently close relationship of the Tuatara with the turtles and tortoises, it seems almost certain that some trace of the parietal eye, which is so strongly developed in the former, will also be discovered in embryos of the latter group; and this is a point to which I would venture to direct the special attention of embryologists.