VARIETIES OF CELL-DIVISION. 1 01 



the division are destroyed, and each protoplasmic body becomes enveloped in 

 a new specially formed cell-wall ; so that finally only these newly formed cell- 

 walls remain after the destruction of the older ones, while the whole complex of 

 cells, produced by repeated bipartition, now consists of individual pollen grains 

 entirely separated from one another; each of which, from its origin and organisa- 

 tion, however, must nevertheless be regarded as a cell. We may here perceive 

 clearly how cells, appearing inside a growing organ as mere chambers, arise in con- 

 nection with reproduction as individual bodies entirely separated from one another. 

 This process is still more conspicuous in the formation of the pollen grains of most 

 dicotyledonous plants, of which an example is represented in Fig. 103. Here the 

 four daughter-cells of a pollen mother-cell arise (at least apparently and so far as 

 concerns the development of the wall) simultaneously, and not by means of a 

 repeated bipartition of the existing cell-cavity. After the preparatory stages to be 

 described later, in which also we can still recognise the principle of bipartition, 

 the protoplasmic body of the pollen mother-cell becomes so constricted around 

 the four nuclei produced by repeated bipartitions, that it assumes in the first 

 place a four-lobed form ; and the relatively very thick wall of the mother-cell 

 growing towards the centre in the form of ledges so projects, that four chambers 

 closed off from one another result {E), in which the nucleated masses of proto- 

 plasm lie. Here then the rounding off of the developing daughter-cells is particularly 

 clear. It becomes still more so in that now, inside each of the chambers of the 

 mother-cell (the so-called special mother-cell), a new cell-wall is secreted by each 

 protoplasmic body, which it retains during further growth, while the chamber- 

 walls arising before and during the division dissolve and disappear V 



The more recent researches in the province of the cell theory have shown 

 that at the foundation of all these different forms of cell-formation there lies a 

 common principle ; that especially the first preparatory stages, which are particularly 

 clear in the behaviour of the cell-nucleus, are everywhere essentially the same ; that 

 it always depends originally upon a bipartition of the substance of the cell-nucleus, 

 and a corresponding grouping of the protoplasm around the centres so arising ; 

 the process being completed by the formation of a new cell-wall. 



What has been hitherto stated especially concerns the external appearances, which 

 are to be recognised clearly enough even with relatively low powers of the microscope. 

 Only recently, with very strong magnifying powers and new micro-chemical reagents 

 and methods, a series of processes have been shown to take place in the interior 

 of the cell-nucleus and the protoplasm, which afford a deeper insight into the 

 behaviour of the living cell contents during the formation of new cells. We owe 

 especially to the researches of Strasburger, Flemming, Schmitz, and others, a very 

 thorough knowledge of the most minute details in the changes of the protoplasm 

 and nucleus during division^. From the statements of the former, as well as from 

 those of a number of zoologists, moreover, the very important fact results that the 



* More details concerning what has been said in the text up to this point are found in my ' Text 

 Book,' the fourth edition of which appeared before the new researches on cell-division. 



^ Of Strasburger's many works on cell-division, mention need be made only of his last 

 book, ' Zcllbildung tiud Zclltheibmg,' Jena, 18S0. Further important are Flemming, 'Beiträge zur 



