336 Theory of Cell-formation [Book ii. 



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 Nageli 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 

 Nageli 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 185 1 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- 



