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 



