A remark, en passant, is due to the considera tion that the corresponding organs and parts of organs of the two sides are not only similar in form and structure, but normally of exactly the same age. The corresponding ossific points of each side make their appearance at pre cisely the same time, and in all respects the original development and subsequent evolu tion and maturation of the embryo proceed exactly part passe. The exceptions alone to this rule of symmetry will chiefly occupy our consideration.
In Man, except perhaps in the very early stages of his existence, exceptions are offered by the heart, great blood-vessels, lymphatics of the trunk, lungs, bowels, liver, spleen, and pancreas, with their appendages. All these parts were in all probability, in the very earliest stages of the embryo, symmetrical ; most of them have been proved by actual 'observa tion to have been so ; it is, however, difficult to conceive this to have been the case with the following other instances of departure from symmetry.
All decussations in the middle line are a-symmetrical. Indeed, if a fibre crosses the median line anyhow but at right angles, there is a departure from symmetry. Con sequently, the decussation of the optic nerves, the decussation of fibres in the me dulla oblongata, and the decussation of white fibres at the Linea alba, are instances of ex ceptions to symmetrical repetition. One is tempted, in the last instance especially, to believe that these decussations are not in the original embryonic pattern or plan ; but that they are developed subsequently, in obedience to subsequent circumstances : the knowledge of the power which circumstances, external or internal, existing at any period of an animal's life, have in modifying the directions of fibres of areolar tissue, and of affecting it in other ways, aids one in this belief.
The most remarkable of the above instances, and that in which the primordial symmetry is most widely departed from, is the heart and the great blood-vessels that are immediately connected with it. The heart first makes its appearance as a mass of cells posited in the middle line, which soon becomes hollow and divided into three compartments, the lower one of which receives the embryonic veins, and is therefore the auricle, whilst the upper one is the commencement of the aorta, or bulbus arteriosus. The middle cavity (the ventricle) becomes bent into a horse-shoe form, so as to bring the auricle and bulbus into apposition. From the latter proceeds a median artery, giving off six arches on each side that surround the space occupied by the digestive canal and converge towards the spine, where they are received by two sym metrical aortae. The vein that enters the auricle is a canal or sinus, that intercom municates between two venous trunks sym metrically placed on each side of the spine, called ductus Cuvieri, the posterior continua tion of which is the cardinal (future azygos and hemi-azygos) veins ; and the anterior, the jugular. One of these ductus is permanent as the superior cava. The omphalo-mesenteric vein, which empties into the auricle, is at this time situated in the middle line between and in front of the cardinal veins, and into it falls the single mesial vena cava descen dens. The precise periods at which these
parts are first discernible, and those at which the changes about to be described take place, will be given in the article Ovum. It is the manner of their metamorphosis alone that bears upon this present inquiry. So far all is symmetrical ; now commence those changes whereby the adult impar arrange ment is eventually effected. The fourth and fifth (from the heart) pairs of aortic arches (the two anterior pairs), and the right one of the second pair, soon disappear. The lefft one of the second pair is permanent as the arch of the aorta, and the third pair persist as the subclavian arteries. The first pair give off branches to the nascent lungs. Meantime, the lower (abdominal) parts of the aorta: have co alesced, and those parts of them which inter vene between the second arch on the left side (the arch of the aorta) and•the third (the left subclavian), and between the first arch of the right side (right pulmonary artery) and the third (right subclavian), disappear. Now we have, on the left side, the first two arches uniting behind to form the left aorta. The first gives off a branch to the lungs, and that portion of it which is beyond this pulmonic branch remains, as the ductus arteriosus, pervious till birth ; whilst that portion which is nearest to the heart remains, in connection with the pulmonic branch, permanent for life as the left pulmonary artery. On the right side the first arch alone, the second having previously disappeared, forms the right aorta, which soon joins its fellow with which it coalesces below. The proxi mal part of this arch is permanent, and re mains in connection with its pulmonic branch as the right pulmonary artery, whilst the part of it beyond the pulmonic branch, together with its continuation, the right aorta, down to its point of junction with its fellow, soon disappears. A septum, meanwhile, has been developed in the ventricle of the heart, divid ing it into right and left ventricles, and an imperfect one in the auricle. The bulbus arteriosus also has been divided by a sep tum, in such a manner that the first pair of arches remains in connection with one division of it, which communicates with one of the ventricles, whilst the permanent left arch of the second pair communicates with the other division, and through it with the other ven tricle. It is most probable that these septa, when first developed, extend from side to side, consequently that the systemic ventricle and aorta are at first in front of the pulmo nary ventricle and artery, and that the heart subsequently undergoes a twist towards the right, carrying the systemic ventricle round behind to its permanent position on the left. In reptiles, in which a great many other of the mammalian fmtal characters are permanent, the root of the systemic is in front of that of the pulmonic artery, and there is not that twining round one another of the great arterial trunks which is met with in man. Thus, then, these, the most unsymmetrical parts of the whole body, can be proved by actual observation to have been originally perfectly symmetrical, and the manner of their attainment to their ultimate unsymmetrical form can be accu rately traced.