336 Theory of Cell-formation [BOOK n.

cell-division the parting-wall must form progressively from without inwards, as in Cladophora, contrary to Nageli'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 opposi- tion 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 N�geli 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 phy- totomists do, is quite unjustifiable, because since 1846, when N�geli 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 unimpor- tant 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-