CAPILLARY VESSELS. 159 
blood constitutes the contents of the primary cells, as well as of 
the secondary ones—the vessels produced by their coalescence ; 
and the blood-corpuscles are young cells which are developed 
in their cavities. 
Thus this last class, comprising tissues, which, in their 
functions, are the most characteristic of the animal kingdom, 
exhibits the same principle of development that we have met 
with in all the others; namely, that cells originate in the 
first place, and that these become transformed into the ele- 
mentary parts of the tissues. The elementary cells in this 
class, however, undergo more essential changes during their 
transformation than those of any previous one. ‘They not 
only do not remain, as in the first two classes, independent, 
that is provided with a special cavity and particular wall; not 
only does a coalescence of the walls of neighbouring cells take 
place, as in the third class, but the cavities of the different 
cells also unite together in consequence of the absorption of 
the coalesced partition-walls of the several cells, so that the 
primary cells cease to exist as distinct objects. It is to a cer- 
tain extent the opposite process to that which occurred in the 
fourth ciass, where, in addition to the prolongation of the cells, 
a splitting of them into several, probably hollow, fibres, a sort 
of division of the cells took place. The type of the trans- 
formation of the primary cells, as presented by nerve, muscle, 
and capillary vessels, is not, however, altogether limited to this 
class, but has been already exhibited by previous classes, and 
even in plants. Some of the pigment-cells have been cited 
before as examples, and the generation of the cells of the liber 
observed by Meyen was brought forward as an instance of 
perfect analogy in vegetables. 
The independent existence of each separate primary cell is, 
no doubt, lost as a consequence of this perfect coalescence of 
several cells ; not so, however, its character as Cell in general. 
On the contrary, several primary cells contribute to form one 
secondary cell, having the full signification of one independent 
cell. Each secondary cell in muscle and nerve forms a closed 
Whole, and the distinction between cell-membrane and cell- 
contents or secondary deposit seems to continue throughout life. 
In this way the nerves bring every part of the body into con- 
