Baer, who about ten years earlier had published
the first part of his celebrated work on embryology, and whose ideas
were rapidly gaining ground, thanks largely to the advocacy of a few
men, notably Johannes Muller, in Germany, and William B. Carpenter, in
England, and to the fact that the improved microscope had made minute
anatomy popular. Schwann's researches made it plain that the best
field for the study of the animal cell is here, and a host of explorers
entered the field. The result of their observations was, in the main,
to confirm the claims of Schwann as to the universal prevalence of the
cell. The long-current idea that animal tissues grow only as a sort
of deposit from the blood-vessels was now discarded, and the fact of
so-called plantlike growth of animal cells, for which Schwann contended,
was universally accepted. Yet the full measure of the affinity between
the two classes of cells was not for some time generally apprehended.

Indeed, since the substance that composes the cell walls of plants is
manifestly very different from the limiting membrane of the animal cell,
it was natural, so long as the wall was considered the most essential
part of the structure, that the divergence between the two classes
of cells should seem very pronounced. And for a time this was the
conception of the matter that was uniformly accepted. But as time
went on many observers had their attention called to the peculiar
characteristics of the contents of the cell, and were led to ask
themselves whether these might not be more important than had been
supposed. In particular, Dr. Hugo von Mohl, professor of botany in the
University of Tubingen, in the course of his exhaustive studies of
the vegetable cell, was impressed with the peculiar and characteristic
appearance of the cell contents. He observed universally within the cell
"an opaque, viscid fluid, having granules intermingled in it," which
made up the main substance of the cell, and which particularly impressed
him because under certain conditions it could be seen to be actively in
motion, its parts separated into filamentous streams.

Von Mohl called attention to the fact that this motion of the cell
contents had been observed as long ago as 1774 by Bonaventura Corti,
and rediscovered in 1807 by Treviranus, and that these observers had
described the phenomenon under the "most unsuitable name of 'rotation
of the cell sap.'" Von Mohl recognized that the streaming substance