DEVELOPMENT OF VEINS. - Are the veins, large and small, originally developed as capillaries, the succeeding changes by which their texture and size are modified taking place subsequently to their being permeable sanguiferous channels ? Whether this is en tirely the case is doubtful : that it is so to some extent is certain; and therefore the development of veins would involve two con siderations — first, the development of capil laries, and secondly, those after-changes by which the vessels cease to be capillaries, and become, in texture and volume, true veins.
The arguments in favour of the view that veins are developed primarily as capillaries, are—that veins, arteries, and capillaries con stitute one continuous system, which renders a community of development antecedently probable ; that arteries and veins are so similar in structure as evidently to possess a common development, which heightens the probability of their development being the same as their connecting link; that the part possessed by capillaries — basement mem brane — is that which we see in other parts capable of giving rise to fresh structures ; that we see all stages between the largest veins and capillaries filled up without a gap; that structures which in an early stage are furnished only with capillaries, and vessels a little removed from them, are found shortly! to contain veins, and that the venous textures become more strongly marked as the parts become more mature ; and that veins but little removed in size from capillaries, differ from them by the addition of structures exactly similar in nature to those which larger veins possess in a greater amount.
The development of capillaries has been ob served, principally, by Schwann* in the tails of young tadpoles, and the germinal mem brane of an incubated hen's egg, by Kollikert, in the tails of batrachian larvae generally, and by Paget in the fcetal membranes of sheep. Their accounts, which my own observations on the tails of batrachian larvm entirely con firm, all substantially agree.
On subjecting the tail of a very young tad pole to microscopical observation, and viewing it with an object glass of a quarter of an inch focal distance, it will:be seen that the vessels possess all the characteristics of very fine capil laries, that is, they possess a delicate, perfectly homogeneous membrane, with nuclei adhering here and there to its internal surface. The two great arterial and venous trunks are elongated posteriorly, as the larvae grow, by throwing out prolongations, which, by joining the embryonic cells accumulated about the extremity of the chords dorsalis, inosculate with them so as to form a continuous cavity.
The first lateral vessels of the tail, which have the form of simple arcs going from artery to vein, are formed by the junction of pro longations, from the caudal artery and vein, with certain elongated or stellate cells in the substance of the tail. From these vessels, which constitute what may be called the pri mary arches,are thrown out projections which, by inosculating in an exactly similar way with neighbouring stellate cells, form secondary capillary arches; and thus the capillary net work continually extends itself in proportion as the tail gets longer and. broader, and at the same time becomes more dense by the forma tion of new vessels between the primitive meshes.
Such are the general appearances seen in the tail of any of the batrachian larva'. These, however, Schwann does not consider so con clusive as the appearances in the germinal membrane of a hen's egg. He says that when the germinal membrane of a hen's egg which has been subjected to thirty.six hours' incuba tion, is placed under the microscope, and the area pellucida examined with a magnifying power of 45Q, the capillary vessels are readily distinguished in it. In some situations they are perfect, and connected with the larger vessel, some are of irregular calibre, having bulgings here and there where two or three channels meet, the intervening portions vary ing in size from that of a fibre of yellow fibrous tis to the full capillary diameter, and permeable to blood or not.
In addition to these capillaries, which form a network of channels of irregular calibre, and give off blind branches, some separate irregular corpuscles are seen which are not connected with the vascular network. These bodies send off blind processes, of various forms, in different directions ; in other words, they are stellate cells. They have a reddish yellow colour, like that of the capillaries of bone, which circumstance alone would be suf ficient to make it probable that they are the primary cells of capillaries in process of forma tion; and this probability becomes a certainty when we perceive that some of these same stellate cells are already connected with the true capillaries, forming with them a common cavity. This is Schwann's description. Of its correctness I cannot speak from my own observation, but of the correctness and ex treme truthfulness of Kolliker's account of the same process in the tails of young batra chia I can, from my own observations, con fidently speak : indeed he has left nothing to be added.