10i 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 

 immediately 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 

 heir's eg^. 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 pi. IV, fig. 11.) The 

 variety in the thickness of this membrane in different in- 

 stances sufficiently explains why we cannot distinguish 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 



