464 THE ORIGIN OF VERTEBRATES 
found in connective tissue. In the first group the physiologist 
recognizes that the central nervous system is connected with. all 
muscular tissues, whether striped or unstriped, somatic or splanch- 
nic, and that such connection is of an intimate character. Further, 
all epithelial cells, either of the outer or inner surfaces, whether 
forming special sense-organs and glands, such as the digestive and 
sweat-glands, or not, are connected with the nervous system. 
Besides these structures, there is another set of organs as to which 
we cannot speak definitely at present, which must be considered 
separately, viz. all the cells, together with their derived organs, which 
line the body-spaces. Whatever may be the ultimate decision as to 
this group of cells, it must fall into one or other of the two main 
groups. 
The members of these two groups are so interwoven with one 
another that either, if taken alone, would still give the form of the 
body, so that, in a certain sense, we can speak of the body as formed 
of two syncytia, separate from each other, but interlaced, of which the 
one forms a continuous whole by means of cells connected together 
by a fluid medium or by solid threads formed in such fluid medium, 
while the other does not form a syncytium in the sense that any cell 
of one kind may be connected with any cell of another kind, but a 
syncytium of which all the different elements are connected together 
only through the medium of the nervous system. 
If we choose to speak of the body as made up of two syncytia 
in this way, we must at the same time recognize the fundamental 
difference in character between them. In the one case the elements 
are connected together only by what may be called non-living 
material; there is no direct metabolic activity caused by the action 
of one cell over a more distant cell in consequence of such connec- 
tion, it is not a true syncytium; in the second case there is a living 
connection, the metabolism of one part is directly influenced by the 
activity of another, and the whole utility of the system depends upon 
such functional connection. 
The tissues composing this second syncytium may be spoken of 
as the master-tissues of the body, and we may express this conception 
of the building up of the body of the higher Metazoa by saying that 
it is composed of a syncytial host formed of the master-tissues, which 
contains within its meshes a system of free-living cells, none of 
which have any connection with the nervous system. This syncytial 
