uninjured), and torn it asunder a little, it was examined with the microscope; the first appearance presented was that of a great quantity of isolated cells, floating about in the surrounding fluid, each of which had its peculiar nucleus. (See plate III, fig. 12.) This easy separation of the cells is never seen in the same degree in the areolar and fibrous tissues, as they are there connected together by the cytoblastema, and by the tough fibres issuing from the cells. These cells of the coat of the aorta vary very much in shape. (See the figure.) Some are round, but most of them oblong, some terminate with a blunt extremity, others are acuminated on two or more sides, others again are prolonged into small processes, which again subdivide, but never extend to any great length. Many of them are somewhat compressed laterally. They all have a granulous aspect, but that appearance, so far as one can judge by rolling the cells about, seems to be referrible to the cell-membrane, and the contents appear to be transparent. The nucleus, enclosing one or two nucleoli, is attached to the interior of their walls. It is sometimes round, at others more or less elongated. These cells have become disengaged from the small portion of the coat of the artery before described. When the preparation itself is examined, many more cells are observed in it, and in addition to them, distinct elastic tissue, consisting of a network of minute, elastic, rough (?) (rauher) fibres, such as are found nearest to the internal coat of the aorta in the adult. (See Eulenburg, de Tela elastica, fig. 9.) It does not, however, present any fibres so thick as those which are found in the external layers of the same part. A blighted nucleus may be recognized here and there in the network. What relation, then, do these cells bear to this still delicate, but so far as regards its characteristic features, perfectly-formed elastic tissue? Analogy would lead us to suppose them to be the primitive formation ; I sometimes also thought, that in rare instances I could observe an immediate transition; that I could see, for instance, one of these cells, furnished with a nucleus, pass continuously on one side into a small portion of reticular tissue, resembling in appearance the undoubted elastic tissue, whilst on its other side it retained its perfect cellular figure. But this occurred so rarely, that I am not enabled to
