west, and until the sun was much above the horizon, the cold was considerable. From this circumstance, and the continual ascent from Murviedro, upon the eastern mediterranean coast of Spain, to a point in the tract Z, called El Puerto, about three leagues beyond Segorvè, on the line of road from Valencia to Teruel, it appears evident that this lacustrine deposite has taken place at a very considerable elevation above the level of the sea; and the same remark may be made with respect to those in the vicinity of Baza and Alhama, as well as the compact limestone observed near La Gineta and Oçana in La Mancha: nor shall I be far from the truth in calculating the surface of the superior beds in the respective basins of Teruel, Alhama, and Baza, at about 2000 feet above the level of the sea. The ground near

La Gineta and Ocāna is still more elevated.

It has been stated, that within the area of each of the lacustrine basins near Baza and Alhama, springs or streams impregnated with muriate of soda existed, the former being called Las Salinas de Vacor, the latter Las Salinas de Malà. Contiguous to the gypsum tract near Aranjuez, which, together with the superjacent compact limestone seen at Ocāna, are, from identity in composition and geological position, presumed to be lacustrine like the same two formations near Baza and Alhama, sulphate of soda has been discovered in sufficient abundance to be employed advantageously in the arts; and a couple of miles higher up the same bank of the Tagus, muriate of soda, in the form of rock-salt, is met with at Villa Rubia, a village whose name is well known to mineralogists as the locality of the glauberite.

The analogies thus pointed out, and the real geological relations of these deposites of salt, I leave to future observers, who may perhaps discover that the high elevation of the plateau of La Mancha is partly owing to a thick and extensive formation of tertiary beds.

* This sulphate of soda was first discovered by a Spanish gentleman of the name of Rodas, who, in consequence, erected a large manufactory at Àranjuez, where it is converted into a carbonate, and sold to soap-makers as a substitute for barilla.

On the Development of the Vascular System in the Fœtus of Vertebrated Animals. Part II. BY ALLEN THOMSON, M. D. late President of the Royal Medical Society. Communicated by the Author. (Continued from former volume, p. 327.)

HAVING in the former part of this essay considered the mode

of formation of the Heart in the different orders of vertebrated animals, I shall now give some account of that of the other parts of the vascular system; viz. of the Blood vessels of the body.

There seem to be two modes principally in which blood vessels are developed; the one, by isolated points and vessels, has been already alluded to, in the account given of the commencement of the circulation, as it occurs on the vascular area of the yolk; the other, taking place after the commencement of the circulation, by the prolongation of loops or folds from vessels already formed, is most easily seen on the transparent parts of the Batrachian reptiles.

The sac of the yolk, or covering which the yolk receives from the layers of the germinal membrane, is the part on which, in all vertebrated animals, the blood and vessels appear to originate, and it is the only part in which, in healthy animals, the formation of blood vessels has been observed to take place independently of the heart or general circulation. During the development of the vascular area (to the detail of which it is now unnecessary to recur), no difference has as yet been observed between the mode of the formation of arteries and veins. The blood appears to circulate sooner in the veins than in the arteries of the area, but, in the early stages of development, these vessels are to be distinguished from one another only by their distribution, and the direction of the currents of blood in them. About the fourth or fifth day of incubation, the coats of the arteries begin to appear thicker than those of the veins, and very soon the external appearance of these vessels affords a character sufficiently distinctive. As far as has been ascertained, there does not appear to be any immediate connexion between the formation of vessels in the area, and that of the heart itself: these processes seem, for a time at first, to go on simultaneously, but independently of one another; and, in

deed, the origin of the heart may not inaptly be compared to that of some of the larger blood vessels. Even when that organ begins to move, no blood enters it from the area: according to Baer, its motion is undulating for a few hours, until it sucks, from the veins immediately adjacent, a portion of their contents, and soon, by a regular contraction of its parietes, propels the blood through its anterior part and the arteries connected with it. At the same time that the vascular area is formed, some vessels are likewise developed in the body of the embryo, in which also the blood and vessels containing it appear to be simultaneously produced. But after the circulation has commenced, the second process to which I have alluded, viz. the prolongation of loops from vessels already existing, seems to be more frequently resorted to for the development of new vessels in the foetus.

This process has been described by Spallanzani *, Fontana †, and Döllinger, as it occurs in the finny tail and external gills of the common frog and water newt. In these animals, the course of the blood is at first very simple. In the early stages of development, there is no capillary network on the tail; but an arterial vessel, continued from the descending aorta, runs below the caudal vertebræ to the end of the tail, where it joins at an acute angle with a returning vein, which, in the abdomen, becomes the vena cava inferior. At a later period, it is well known that the tail of these animals is covered by a network of minute vessels, which communicate with the primary artery and vein. Through this network the blood is spread over the whole surface of the tail. The development of these vessels has been shewn to be owing, not to their formation separately in the parenchyma of the tail, but to the prolongation of communicating vessels formed between the primary trunks. The communicating branches at first pass directly from the artery to the vein, but in the progress of development I have observed them to become gradually longer, and extend themselves from the middle to the lateral expanded parts of the tail: other loops are formed in succession from the newly generated vessels, and new ones again from them, till, in the course of ten or more

Experiments on the Circulation of the Blood, &c. + Reil's Archiv für die Physiologie, B. ii. S 480. Denkschriften der Königl. Akad. München. B. vii.

days, the whole of the finny part of the tail is covered by beautiful and minute arteries and veins. The loop of vessel when short and newly formed, has at first more the appearance of artery than vein, as the blood passes through it in jerks: as the loop elongates, however, and new branches proceed from it, the blood moves in jerks only in that part of the loop which communicates with the arterial trunk, while in the part connected with the returning vein, the motion of the stream of blood becomes uniform.

Rusconi* has shewn very beautifully, that vessels are thus looped out during the development of the gills of the aquatic salamander. I find that nearly the same appearances present themselves in the gills of the frog, as well as in the extremities of the salamander. The anterior extremities of the salamander, when they first begin to sprout, form two small tubercles situated behind the head, altogether destitute of circulating blood. Shortly after the appearance of these tubercles, a single vessel is seen winding round their extremities, which returns to the body without giving off any branches. The parenchyma of each of the toes, as it buds out from the end of the limb, receives a small loop from the original vessel. Communicating branches are likewise thrown across at the joints, and, as the limb becomes larger, numerous capillary vessels are formed in the same way as the primitive trunks.

Spallanzani, and some other observers, have noticed similar appearances in the extremities of the chick, when they begin to be formed; and the same may be seen in those of the rabbit, and of some other mammiferous animals; from which there appears every reason to believe that, after the circulation of the blood has commenced, the development of new vessels from those already formed, takes place principally by means of loops in Warm as well as in Cold blooded animals.

I ought now, in conformity with the plan previously laid down, to proceed to treat of the development of the individual parts of the vascular system in vertebrated animals; but it must appear obvious, that a detailed account of the development of

* Amours des Salamandres Aquatiques, et développement, &c. See Plate II. Figs. 8, 9, 10, 13, H h.

all, or even of the more important blood vessels of the body, besides being too extensive a subject for our present limits, would prove uninteresting from the want of connexion existing between the facts already ascertained. I shall therefore confine myself for the present to one branch of the subject only, viz. the Development of the Blood vessels more immediately connected with Respiration in the foetal or adult animal. This branch of the subject, besides being the most nearly allied to that treated of in the first part of the essay, is rendered one of the most interesting to comparative anatomists, not only by the diversity of the form and by the number of the organs which appear to carry on the respiratory function in the foetus of vertebrated animals, but also by the singular analogies in the structure of these animals which the study of the development of their respiratory organs points out both in their transitory and permanent condition.

The principal organs which appear to perform a respiratory function in the fœtus, or which, being formed before birth, are destined for the respiration of the adult animal, may be enumerated in the following order, being that in which they succeed one another, either in individual animals, or in the different orders of the class Vertebrata. 1. The sac of the Yolk; 2. The External Gills; 3. The Internal Gills; 4. The Allantois; 5. The Placenta; 6. The Lungs *.

Some of these, as well as other parts of the ovum, have received so many different names, from the various authors who have described them, that it appears necessary to anticipate a little, and to give a few of the synonymes by which they are generally known in the different orders of vertebrated animals.

1. The sac of the yolk is generally known by this name in Fishes, Reptiles, and Birds. We have only in these animals to guard against confounding the sac of the yolk or covering given to this part by the layers of the germinal membrane, with the proper envelope of the yolk which exists before development commences, and encloses it while in the ovarium. In Mammalia, this part is most frequently called the Umbilical Vesicle, and sometimes the tunica erythroides. The distinctive character of the sac of the yolk is, that it remains connected or communicating with the intestine during some period of fœtal life, and has mesenteric arteries or veins, or both, ramified on its surface.

2. The Allantois, (a name derived originally from the vesicular membrane of mammalia) does not exist in the foetus of aquatic animals, such as that of fishes and batrachia. In adult batrachia it forms the urinary bladder,