ing but a continual formation and differentiation of cells, from the
moment of its appearance up to the time when, through the development of
the serous and mucous layers of the blastoderm, the foundation is given
for all the tissues subsequently appearing: we have found this common
parent of all tissues itself to consist of cells; our next task must be
to demonstrate not only in this general way that tissues originate from
cells, but also that the special formative mass of each tissue is
composed of cells, and that all tissues are either constituted by simple
cells or by one or other of the manifold kinds of modified cells" (p.
71). Five classes of tissue can be distinguished, according to the
extent and manner of the modifications which the cells composing them
have undergone. There are first of all independent and isolated cells,
such as the corpuscles of the blood and lymph, not forming a coherent
tissue in the ordinary sense. Next there are the assemblages of cells
lying in contiguity with one another, but not in any way fused; examples
of this class are the epidermal tissues and the lens of the eye. In the
third class come tissues the cells of which have fused by their walls,
but whose cell-cavities are not in continuity, such as osseous tissue
and cartilage. In the tissues of the fourth class, comprising the most
highly specialised of all, not only are the cell-walls continuous but
also the cell-cavities; to this class belong muscle, nerve and capillary
vessels. A fifth class, of rather a special nature, includes the fibrous
tissues of all kinds. This is the first classification of tissues upon a
cellular basis, and it marks the foundation of a new histology which
took the place of the "general anatomy" of Bichat. The exhaustive
account which Schwann gives of the structure and development of the
tissues in this section of his book constitutes the first systematic
treatise on histology in the modern sense, and it is still worth
reading, in spite of many errors in detail.

Schwann found it easy to demonstrate the cellular nature of the tissues
of his first three classes. With the other two classes he had more
difficulty. Fibres of all kinds, he considered, arose by an elongation
of cells, which afterwards split longitudinally into long strips,
forming as the case might be white or elastic fibrous tissue.
Muscle-fibres and nerve-fibres were formed in a totally different way,
by coalescence of cells; each separate mus