cell-division the parting-wall must form progressively from
without inwards, as in Cladophora, contrary to Nägeli's and
Hofmeister's correct statements, that cases also occur of
simultaneous formation at every point of the surface of the
partition-wall. As usual, however, von Mohl rested his opposition on a good observation, and showed that in the case
of the formation of pollen in dicotyledonous plants it is
possible to burst the membrane of a mother-cell in the act
of dividing, and set free the protoplasm when it is already
deeply divided into the four parts, and so to see the half-formed partition-walls; but this only proved that such was the
process in the cases observed, the formation of the partition-walls being simultaneous in others. It may be mentioned
in this place, that the idea of special mother-cells in the
formation of pollen introduced by Nägeli in 1842 was in entire accordance with the condition of the science at the
time, since he meant by the term the laminae of membrane
formed during the successive divisions of the mother-cell.
To call these still special mother-cells, as some modern phytotomists do, is quite unjustifiable, because since 1846, when
Nägeli propounded his theory, the word cell, as we have
seen, no longer designated the mere membrane but the
whole body of the cell, while the expression special mother-cell rests on the older phraseology, in which cell and cell-membrane are identical.
The additions made to the doctrine of cell-formation during the greater part of the twenty years after 1851 were unimportant in comparison with the mighty development which it had experienced during the preceding ten years. These years had indeed been marked by the greatest possible activity and fruitfulness in results in all parts of botanical study. By the labours of Unger, von Mohl, Nageli, Braun, and Hofmeister, not only were the foundations laid for a true theory of cells, but the details were worked out, and the conceptions connected with them finally cleared up. Text-
