Art. III. — On the Occurrence of Two Unusual Blood-vessels in Hyla aurea.
[Read before the Otago Institute, 9th July, 1918; received by Editor, 17th December, 1918; issued separately, 14th May, 1919.]
In former days, when fixity of species was a tenet of biologists, any unusual occurrence in the anatomy of animals was spoken of as an “abnormality”; but nowadays biologists are familiar with the fact that no two individuals of a given species are absolutely identical in all their parts—every organ, both external and internal, may present some more or less profound difference when compared with other individuals, and these differences are known as “variations.” In the blood-system, for example, although the main
blood-vessels conform to a type in all the individuals of a species or even genus, yet the mode of branching, the number of the branches, their size and extent and distribution, are very rarely identical over a series of individual specimens. So it is with other organs or systems of organs: some of these “variations” are in the direction of loss of parts, or they may be of additions of parts—new structures which suddenly appear without any transition between them and the usual state of the organ. We call these” mutations” if they are hereditary, though in a large number of cases it is impossible to determine whether this is so or not. But others of the variations from the usual adult structure are due to the persistence of conditions which are present in the embryo or in some lower member of the group to which the species belongs, and which are usually lost during the development, so as to be absent in the adult. Such persistent embryonic structures are always of great interest. Two such cases were met with amongst the adult frogs, Hyla aurea, dissected in my laboratory during this session. Both of these conditions seem to be extremely rare, and, so far as I have been able to consult the literature at my disposal, they appear to be unique.
Persistence of The Third Branchial Aortic Arch.
On the 28th March one of my students called my attention to the presence in the specimen she was dissecting of four arterial arches on each side, in place of the normal three. The frogs had been injected so that the students should be able to trace out the arterial system, and in the
Fig. 1.—Arterial aortic arches (X 2) showing persistent third larval arch. m, portion of petrohyoid muscle; l, small laryngeal artery.
Fig. 2.—Visceral veins in abnormal frog (from drawing by E. W. Hunt). The heart is turned forwards to show the sinus venosus and the hepatic veins. lv, the unusual paired lateral (abdominal) vein; uv., the vesicular vein entering the hepatic portal.
case under consideration the injection, performed by my assistant, Miss W. Farnie, was particularly successful, so that even small branches were distended with the injection mass, such as the laryngeal and pharyngeal arteries. The student had dissected out the arteries on the animal's right side with great care, and only the internal carotid had been ruptured. The other side she had scarcely touched when she directed my attention to its condition.
The arrangement is as follows: The carotid and systemic arches are normal, but the pulmo-cutaneous arch bifurcates close to its origin, the posterior branch of which has the usual relations dividing into the pulmonary artery and the cutaneous artery. It is the anterior of the two branches which is exceptional, and appears to be the persistent third branchial arterial arch of the tadpole. This anterior branch arises from the base of the pulmo-cutaneous soon after it leaves the synangium: it is equal in diameter to this arch, and runs parallel with the other arches nearly to the point at which the last arch divides to form the pulmonary artery and the cutaneous artery. But as it approaches this cutaneous artery its diameter decreases and it bends backwards towards the fourth arch, to which it is joined by a very slender vessel. It then bends forward again and is continued into the cutaneous artery, alongside which runs as usual the petrohyoid muscle, to which it gives off twigs. The relation of this third arch to the cutaneous artery would suggest that the latter is derived from it, were it not for the precise account of the development of the latter given by Marshall. At the first bend of the arch is an angle as if a vessel or ligament passed forwards to the systemic arch, but I can find no trace of this. There is no connection between this third arch and either the systemic arch or the dorsal aorta.
On the left side I find that the condition of affairs is essentially the same, but the third arch is much more slender than on the right side. Less injection has penetrated the vessel, which suggests that possibly some resistance is exerted at the connection between it and the cutaneous artery. Nevertheless, the connecting vessel is distinctly red with injection, but is much narrower than its basal region. As on the right side, this third arch bends backwards (more abruptly than is shown in the drawing, for it is better seen when the arches are stretched apart) in order to reach the cutaneous artery, which on this side is normal and of equal diameter throughout its course.
The delicate pharyngeal artery, from the systemic arch, is plainly visible in the specimen below the third arch, but I have omitted it from the drawing for the sake of clearness.
According to Marshall,* during metamorphosis “the third aortic arch in the third branchial arch of the tadpole atrophies altogether. In young frogs of the first year it loses its connection with the aorta and then gradually shortens up, the distal end becoming a solid cord, and the proximal or cardiac part retaining for a time its lumen. Before the end of the first year this vessel has entirely disappeared.”
In the larva this third afferent arch goes, of course, to a gill, and has no connection distally with the fourth arch: it is this union on the ventral surface that is rather puzzling in the present case, and especially the very slender union between the cutaneous artery and the parent fourth arch. It raises the question whether the ontogeny of the frog is really a true recapitulation of the phylogeny of the Anura, or whether the cutaneous artery is originally derived from the third arch, which in the embryology of those species of frog that have been studied has undergone some modification, leaving the third to become connected with the fourth arch at some stage in the history.
The cutaneous artery seems to be peculiar to the Anura, as no reference is made to such an artery in any description of the anatomy of salamander
[Footnote] * A. M. Marshall, Vertebrate Embryology, p. 178, 1893.
or newt to which I have access (e.g., Bronn's Thierreich). In the salamander, as is well known, both the third and fourth arches persist, but unite with the systemic to form the dorsal aorta.
As I have noted, the third arch in the case under consideration is not connected on either side to the dorsal aorta or with the systemic arch.
2. Paired Lateral Abdominal Veins.
A frog dissected by one of my students on the 4th April rather worried him because there was no “anterior abdominal vein,” and so he called my attention to it. I was surprised to find that in place of this median vein this frog possessed a pair of laterally situated veins each of which, arising from the femoral vein of its side and running forwards in the body-wall, quite laterally entered the precaval (anterior vena cava) of its side. On its way it received two musculo-cutaneous veins, which normally in this species enter the anterior abdominal vein at the level of the tendinous intersections of the rectus abdominis muscle. These lateral veins had no relation to the portal system, but the vesicular vein from the urinary bladder passed forwards to enter the hepatic portal at the spot at which the abdominal vein normally does so.
I can find no record of exactly this arrangement as occurring in the frog, though cases of a right or left vein of somewhat similar relations anteriorly have been described.
Buller* found an abdominal vein which after a normal course from the hinder part of the body as far as the liver, to which it sent a small branch, bent outwards to the right side and entered the right superior vena cava, or, as the figure shows, the subclavian.
The next case is that of Woodland,† where the abdominal vein is median posteriorly but passes outwards to the left precaval (or subclavian). It gives no branch to the liver.
In the next year O'Donoghue‡ described a frog with an abnormal heart and with an abdominal vein similar to that described by Woodland.
These are the only instances of an abnormal condition of the abdominal vein which I can find. In each of them the hinder end arises quite normally from the union of the two femoro-abdominal or “pelvic” veins. Each of these authors refers to the condition in Ceratodus, while Woodland carries the comparison back to the paired lateral veins of the dogfish, which homology was first suggested by Hochstetter in 1894.
In the present case this resemblance is very evident and precise. Here, too, is an instance of the persistence of a larval condition, though with certain differences in detail. Marshall‡ writes thus: “The anterior abdominal vein is at first paired and is in connection not with the liver, but with the heart. The pair of vessels appears first in the ventral body-wall, extending backwards a short distance from the sinus venosus; they soon extend farther backward and acquire a communication with the veins of the hind legs and of the bladder. At a later stage the two veins unite at the hinder end in front of the bladder, while farther forward the vein of the right side disappears and the left one alone persists; later still the
[Footnote] * A. H. R. Buller, Journ. Anat. and Physiol., vol. 30, p. 211, 1896.
[Footnote] † W. Woodland, Zool. Anz., vol. 35, p. 626, 1910.
[Footnote] ‡ C. H. O'Donoghue, Zool. Anz., vol. 37, p. 35, 1911.
anterior abdominal vein loses its direct connection with the sinus venosus, and acquires a secondary one with the hepatic portal system.”
In the European frog Bombinator, which is closely allied to the New Zealand frog Liopelma, Goette* had already described a similar course of events, while Hochstetter† found in the larva of the salamander that the two abdominal veins are at first separate for the greater part of their length, but unite near the liver to form a median vein which enters the left precaval vein; but with the absorption of the yolk this communication is lost and a new one established with the hepatic portal system.
The occasional persistence in Rana of either a right or a left connection with the sinus venosus by way of a precaval has already been emphasized by Woodland and O'Donoghue.
That the present instance is not an exact recapitulation of the larval condition is seen by the fact that the anterior end of the lateral vein on each side is not connected with the sinus venosus but with the precaval, which is apparently an earlier condition, as seen in the Elasmobranchs.
[Footnote] * A. Goette, Entwickel der Unke, 1875.
[Footnote] † F. Hochstetter, Morphol. Jahrbuch, vol. 21, p. 19, 1894.