pharynx. At first, this vessel divides itself only into two branches situated immediately behind the mouth, which, passing round the pharynx, reunite with one another on its upper part, below the vertebral column, to form the descending aorta. Shortly afterwards, a farther subdivision of the aorta takes place, by which four new vascular branches (m) are formed on each side of the pharynx, behind the one which appeared first, and leading from the ascending to the descending trunks of the aorta. These vessels, the primitive branchial arteries, are at first nearly of a uniform diameter, and do not divide into any branches in their course round the pharynx. Each artery runs along one of the branchial plates or hoops, (Fig. 2, I).

As development proceeds, four transverse clefts, the commencing branchial apertures, appear between the branchial hoops on the lower and lateral parts of the pharynx (Figs. 2 and 3, 0). The four posterior.hoops become stronger, more cartilaginous, and project farther from the side of the œsophagus, and, at the same time, little leaflets or tubercles, the rudiments of those which afterwards form the comb-like fringe of the gill, begin to be formed on their external sides. Each of these leaflets, soon after its first appearance, is furnished by the large vessel of the hoop with a little artery and vein, probably formed in the same manner as the vessels of the tail and gills of the salamander already alluded to. The number and size of the leaflets gradually augment, and, at the same time, each of the vascular arches is farther subdivided, till at last, when the structure of the gill is perfected, instead of the single arterial vessel, which at first winds round each hoop, there are formed a branchial artery and vein, the capillary vessels of which join by a multitude of minute ramifications on the surface of each of the leaflets. It has thus been shown by Rathke, that the branchial vein is originally a continuation of the trunk of the branchial artery. While those changes are taking place, the anterior vascular arch on each side, not contributing, like the posterior, to form the vessels of the gills, gives off an arterial branch (Fig. 1, t) at its anterior and convex part, which rises to the head, and corresponds with the carotid artery of higher animals. This vessel now no longer communicates with the ascending aorta, but is supplied with blood by the posterior

1

part of the arch which joins the first pair of branchial veins at the place where the root of the aorta begins.

The anterior branchial plate, or that situated between the mouth and the foremost branchial apertures (Figs. 2, 3, λ) instead of being covered by the fringe of leaflets proper to the gills, projects farther backwards in the neck, and begins to overlap the branchial plates posterior to it (Fig. 4, 5 λ). According to Rathke, the anterior part of this plate forms the lower jaw; the posterior part, continuing to extend itself backwards, forms the opercular covering of the gills.

The branchia of the Rays and Sharks, again, consist of five pairs of double branchial plates, from the edges of which rows of leaflets are suspended; but, so far as I know, the mode in which they become developed has not been observed. It would be interesting to know in what manner the opercular fold and openings of the gills are formed in these animals, as the smallness of the lower jaw and the absence of the lateral processes of the lingual bone, seem to indicate that only five branchial plates originally exist in the fœtus, and that all of them become developed in the formation of the branchia of the adult animal.

The foetus of cartilaginous fishes besides possessing a yolk-sac, in the vessels of which a large quantity of blood is spread out, are provided, during a considerable period of their fœtal life, with other organs, by means of which the aëration of their blood is promoted. The External Gills or branchial appendages were known to the late Dr Monro*, and are described as such by him in the skate, (see Fig. 6, A. H). The connection of these appendages with the hoops of the internal gills, was, however, first shewn by Dr Macartney of Dublin in the fœtus of the shark. They consist, according to this anatomist, of five bundles of tender filaments hanging from each side of the neck. These bundles appear to arise from the external integuments, but are really attached to the internal gills on the inner side of the branchial apertures +.

In some beautiful specimens of the external gills of the Squalus Catulus and S. maximus, which I had lately an opportunity of seeing in the Museum of the College of Surgeons, Lon

* Monro on Fishes, plate XIV. + Journal de Physique, Fevrier 1818.

OCTOBER-DECEMBER 1830.

G

don, collected by Sir E. Home and Mr Clift (see sketch, Fig. 7)., I could easily perceive that each of the filaments of which the five bundles were composed consisted of a single fold or loop of véssel, covered by a thin membranous layer prolonged from the integuments. (Fig. 7, A.)

Development of the Respiratory Organs in the Fatus of Reptiles.

1. BATRACHIA. The development of the respiratory organs in the foetus of these animals is peculiarly interesting on account of the transition which they undergo from an aquatic to an aerial condition; and the observation of the changes of structure which take place during their transformation, appears to have illustrated, more clearly perhaps than that of any other class of animals, the relations of the respiratory and vascular organs to one another.

Aquatic Salamander.-Shortly after the foetus or larva of the Aquatic Salamander leaves the egg, its blood is exposed to the influence of the surrounding water on the surface of the rudimentary intestine, or part corresponding with the sac of the yolk in other animals. The vena cava returning from the tail, on arriving at the posterior part of the intestine, gives off a large branch (Fig. 8, y'), which is joined by numerous small vessels spread over the lower surface of the abdomen. The small vessels are again united into one venous trunk (y), situated near the auricle of the heart, on the left side of the body, where they pour into the heart (a) a large quantity of blood which passes over the abdomen, along with that from other parts of the body.

The liver (c) is formed in the batrachia, as in cartilaginous fishes and the higher orders of animals, by the subdivision of the venous trunk conveying the blood from the yolk to the heart.

As the development of the foetus proceeds, its finny tail, as well as the greater part of the integuments of the body, are covered by minute ramifications of vessels, which must contribute materially to aërate more perfectly the blood. As these ramifications become more numerous, the quantity of blood sent over the abdomen becomes less, the liver (Fig. 8. c) increases in size, the proper mesenteric vessels are formed, and the trunk of the proper vena cava augments, and carries proportionally a greater quantity of blood directly from the tail to the heart. At the

same time, the formation of the external gills, or more special respiratory organ of the foetus of these animals, commences.

The rudiments of the external gills, very similar in their commencement to the branchiæ of the foetal fish, are to be found at a very early period. Some days before the fo tus of the Salamander quits the egg, they are indicated by four transverse opaque bands on the fore part of the body, or pharyngeal portion of the intestine. These transverse bands by their farther development form branchial hoops on each side of the neck. The integuments then begin to bud out at the upper and lateral parts of these hoops, so as to form three small projections or folds of the skin, placed severally opposite the interstices between the hoops. According to Rusconi *, before these parts, which are the commencing external gills, receive any vessels, the distribution of arteries in the neck is very simple, and analogous to that noticed by Rathke in the fœtus of the osseous fish. The arterial vessel prolonged from the bulb of the aorta advancing forwards below the neck, is divided into four pairs of smaller vessels, four of which passing round each side of the pharynx, unite with those from the other side below the vertebral column, to form the descending aorta; each pair of vessels, as it is given off by the aorta, passes along one of the branchial hoops. Rusconi has ascertained, and I have repeated his observations with the same result, that these vessels are at first quite simple in their course round the pharynx, and do not give off any branches.

At the time when the embryo comes out of the egg, the little processes which constitute the commencing external gills, are considerably elongated, and are each supplied with a loop of vessel from the outermost part of one of the branchial vascular arches (Fig. 8, m). As the stalk of the gill sprouts out farther from the side of the neck, it acquires considerable length, and the loop of the branchial vessel, consisting simply of an outgoing and returning branch, is prolonged into it. When the primitive stalks of the gills have acquired greater length, new buds of the parenchyma begin to appear on their lower sides, forming the commencing leaflets of the gill (Figs. 8 & 10, h),

Amours des Salamandres Aquatiques, et Développement du Tétard, &c. ; and in his Descrizione Anatomica degli organi della circolatione delle larve, &c.

and opposite to each of these secondary buds a new branch of vessel is formed by the passage of the blood, directly across the primary stalk, from the outgoing to the returning vessel. As the new buds of the gills become longer, these communicating branches between the primary vessels are produced along with them; new communicating branches are thrown out in their course, while, at the same time, new buds are formed on the primary stalk of the gill. There are generally about thirty of these leaflets on the gills of the larva, at the time that they have attained their full development, which, according to Rusconi, is about the 40th or 45th day; the vessels are then still more minutely ramified on the surface of the leaflets, and as they are almost quite transparent, the circulation of the blood through them forms a truly beautiful spectacle. In this animal, therefore, as in the fish, by tracing the development of these vessels, it is easy to perceive that the branchial arteries and veins are only subdivisions in the course of the aorta itself.

At the period when the gills are fully developed, the distribution of the vessels rising from the heart is the following (Fig. 9): The three foremost pairs only of the branchial vascular arches convey blood to the gills (m). At the root of each gill the arterial or outgoing vessel communicates directly, by a short branch, with the vein or returning vessel, so that a considerable portion of the blood propelled into the branchial arteries, along with the whole of that in the fourth or posterior pair of arches, which gives no branches to the gill, is carried directly into the descending aorta (rs). From the communicating branch at the root of the foremost gill there arises a small artery (t), which is distributed on the parts surrounding the hyoid bone. The parts near the temporal bone receive an artery from the root of the second gill (t), and,the vertebral artery is given off near the place where the whole of the branchial arches unite to form the roots of the descending aorta (r). The anterior extremity, like the pectoral fin of the fish, receives its vessels from the mammary artery (u), which arises along with the coeliac, mesenteric, &c. from the descending aorta.

While the external bra ichiæ are developed, the internal arches upon which they are supported become firmer and more cartilaginous. They are attached anteriorly to the hyoid bone,