CAPILLARY VESSELS. 155 
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-deve- 
loped 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 in- 
vest the capillary vessels. It is true that epithelial cells oceur 
im 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 pro- 
ject, 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 le 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 cur- 
rent, 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 ordi- 
nary 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 
