lOO 



LECTURE VII. 



make a fundamental distinction impossible. It is more a matter of an external 

 difference concerning the form of the daughter-cell, than an essential difference 

 in the process of formation itself. The chief difference between the so-called 

 free cell-formation and ordinary cell-division lies especially in that, in the former, 

 the developing daughter-cells appear not as mere segments of the mother-cell, 

 but as rounded off individuals; and whether in this a part of the protoplasm 

 of the mother-cell remains unemployed or not, may be considered as unessential 

 for the cell-formation itself, although specifically important for its purpose at the 

 time. We have, for example in Fig. 98, a case of cell-division where numerous 

 daughter-cells, at first polyhedral but subsequently rounded off, are developed from 



V\C. 103. — Althiza rosea. — Division of the pollen mother-cells into four, A — I- ^ /'and G a tetrad. The membrane 

 of the 'special mother-cell" has burst under the influence of water, and allowed the protoplasmic body of the youn;; 

 pollen cells to escape. H a fully developed pollen grain seen from without, and magnified to the same extent. 



the protoplasm of a relatively large mother-cell ; here the first processes come under 

 the same category as ordinary cell-division, while the later ones resemble free cell- 

 formation. On the other hand, we find in the origin of the pollen grains in Fig. loi, 

 in the ordinary repeated bipartition of a mother-cell, with the simultaneous secretion 

 of a cell-membrane and rounding ofi' of the mother-cell itself, a case in which the cell- 

 division presents itself in the form previously described as a development of chambers 

 inside chambers already existing; only that the chamber-walls, as they rapidly 

 grow in thickness, round off the cavities enclosed by them. To be sure, there 

 appears later a very conspicuous difference from the ordinary bipartition of 

 cells in vegetati\'e growing organs, in so far, that in this case the walls formed in 



