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Rh structure, they have arrived at a more complicated stage of their formation, and I regard such fibres as distinct from their cell-membrane.

2. Very distinct cell-nuclei occur at different spots upon the walls of the capillaries, both of the young and fully-developed tadpole. They appear to lie either in the thickness of the wall, or on the internal surface of the vessels, on which they often form a projection. (See fig. 11.) They admit of a double explanation. They are either the nuclei of the primary cells of the capillaries, or nuclei of epithelial cells, which invest the capillary vessels. It is true that epithelial cells occur in vessels which have a great resemblance to capillary vessels, if they are not actually such, as may be very distinctly seen in the vessels of the membrana capsulo-papillaris in a foetal pig of from four to six inches long, where some of them project, in the form of half-spheres, into the cavity of the vessel ; but there were no epithelial cells perceptible surrounding the nuclei in the capillaries of the tadpole’s tail. On the contrary, these nuclei frequently seemed to lie free upon the internal wall of the vessel, and must have been much more abundant had they been nuclei of epithelial cells. That these are the nuclei of the primary cells of the capillaries is, therefore, most probable, although this exclusive argument by no means decides the question.

3. In the tail of very young tadpoles, the capillary network presents, besides the ordinary cylindrical canals which have an equal diameter, and in which the blood flows in a regular current, other vessels of an irregular form. Unfortunately I neglected to make a drawing of them; they accord, however, in all essential particulars with the capillaries of the germinal membrane of the hen’s egg represented in pl. IV, fig. 12, except that the meshes of the vascular network are much larger in the tail of the tadpole. They are not regularly cylindrical. They are generally widest in situations where branches are given off, sometimes wider even than the ordinary capillary vessels. (See a, 6 in figure 12.) The branches diminish very rapidly as they leave those broad parts, and widen again as they approach another dilated portion. They present every degree of narrowing from vessels in which it could scarcely be remarked, to those which are reduced so