LAW OF CONTINUOUS DEVELOPMENT. 55
are still a few spots in the body where absolute demonstration has not yet been afforded, the principle is nevertheless established, that in the whole series of living things, whether they be entire plants or animal organisms, or essential constituents of the same, an eternal law of continuous development prevails. There is no discontinuity of development of such a kind that a new generation can of itself give rise to a new series of developmental forms. No developed tissues can be traced back either to any large or small simple element, unless it be unto a cell. In what manner this continuous proliferation of cells (Zellenwucherung), for so we may designate the process, is carried on, we will consider hereafter; to-day, my especial object only was to deter you from assuming as the groundwork of any views you might entertain with regard to the composition of the tissues, these theories of simple fibres or simple globules (elementary fibres or elementary globules). —
If it be wished to classify the normal tissues, a very simple point of view, founded upon marked characteristics, offers itself, upon which their division into three categories may be based.
We either have tissues which consist exclusively of cells, where cell lies close to cell—in fact, cellular tissue in the modern sense of the word—or we find tissues, in which one cell is regularly separated from the other by a certain amount of intermediate matter (intercellular substance), and, therefore, a kind of uniting medium exists, which, while it visibly connects the individual elements, yet holds them separate. To this class belong the tissues which are now-a-days generally comprehended under the name of connective tissues (Gewebe der Bin- desubstanz), and of which what was formerly universally called cellular tissue constitutes the chief portion Finally, there is a third group of tissues, in which the
