difficult to understand and in some respects quite peculiar,
von Mohl believed that he saw in many cases in the higher
plants also between the sharply-defined membranes, which
bound the cell-spaces and which he regarded as the entire cell-membranes, a substance in which the cells are imbedded, for
such is its appearance when it is largely developed; when it
lies in small quantity only between cells in close apposition, it
looks like a thin layer or cement. After Meyen in his 'Neues
System,' pp. 162, 174 had declared against this view in 1837,
von Mohl too abandoned it more and more, and afterwards
limited the occurrence of intercellular substance to certain
cases, being convinced that much that he had before taken
for it consisted only of layers of secondary thickening, between
which he still saw the primary lamina of the cell-membrane.
The theory of intercellular substance was taken up and further
developed by other phytotomists, by Unger especially in the
Botanische Zeitung for 1847, p. 289, and afterwards chiefly by
Schacht; Wigand came forward as an opponent of it in 1854 in
his 'Botanische Untersuchungen,' p. 65, and logically following
out von Mohl's theory of the cell-membrane declared the thin
layers of intercellular substance as well as the cuticle, which had
been first correctly distinguished by von Mohl, to be laminae of
primary cell-membrane, the substance of which had undergone
profound chemical change. These ideas also of the intercellular
substance and the cuticle assumed an entirely different aspect
when Nägeli introduced his theory of intussusception.
The limits imposed on this history render it necessary to be content with these indications of von Mohl's share in the working out of the theory of cells in its connection with the structure of the solid framework of cell-membrane; we shall return again to his observations on the formation of individual cells.
5. Forms of tissue and comparative anatomy. Phytotomy up to 1830 had been weak in its classification of tissues, in its ideas as to their arrangement, and consequently in its histological terminology; the inconvenience arising from
