154 CAPILLARY VESSELS. 
3. Capillary vessels. 
Plate II, fig. 9, represents two stellate pigment-cells, which 
have coalesced at a. In that instance two cells had been gene- 
rated at some distance from one another, their bodies may still 
be distinguished as two spots somewhat thicker than the rest 
of the structure. These cells became elongated on different 
sides into hollow processes, which, like the cavities of the bodies 
of the cells, are filled with pigment. Two processes of the two 
cells came into contact at a, and then coalesced, the separation 
at the point of union appears to have been absorbed also at the 
same time, so that the cavities of the two cells communicate 
iunmediately with one another; at all events there is no appa- 
rent interruption to the pigment, which forms the contents of 
the cells and their prolongations. (See page 78.) Now, if we 
imagine several such stellate cells to be developed on a large 
surface at similar distances from one another, and the several 
prolongations issuing from each separate cell to coalesce with 
those issuing from the other cells, in the manner represented 
in the figure at a, the result will be a network of canals ex- 
tending over the entire surface, and all communicating with 
each other. The size of the meshes of the network is deter- 
mined by the distance of the cells from each other, and by the 
number of the prolongations issuing from each cell. Such, 
then, appears to be the process by which the capillary vessels 
are formed. 
The observations, on which this mode of formation of the 
capillary vessels is based, were made partly on the tails of very 
young tadpoles, and partly on the germinal membrane of the 
hen’s egg. They are as follows: 
1. The capillary vessels, in the tail both of the fully-deve- 
loped and young tadpoles, are seen to be surrounded by a 
thin, but distinctly perceptible membrane, which does not ex- 
hibit any fibrous arrangement. (See pl. IV, fig. 11.) The 
variety in the thickness of this membrane in different in- 
stances sufficiently explains why we cannot distmguish it in all 
capillary vessels, just as we cannot detect the cell-membrane 
even in the blood-corpuscles, although there can be no doubt 
of its existence. Where the capillary vessels exhibit a fibrous 
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