CELL-DIVISION. 41 



nent part taken by the vacuoles in the former. Although the mem- 

 branes of these vacuoles may not, at first, be exactly similar to plasma 

 membranes, they are undoubtedly converted into them. Since we 

 assume that the plasma membrane is largely of a kinoplasmic nature, 

 and attribute to it something of a morphological rank in the cell, it 

 may not be wholly fanciful to suggest that the limiting membrane of 

 a vacuole may be developed into a real plasma membrane, and that 

 this actually takes place in the plants in question. 



CELL-DIVISION IN DICTYOTA AND STYPOCAULON. 



There is yet another method of cell-formation which has been 

 observed in certain of the brown algae that differs materially from the 

 process of cleavage already described. There are no kinoplasmic 

 connecting fibers by which a plasma membrane may be formed, nor 

 is it a cleavage such as has been described for certain fungi. 



The plasma membrane, or cell-plate, seems to be formed directly 

 out of the apparently undifferentiated framework of the cytoplasm. 

 This type of cell-formation has been observed in such Phaophycece 

 as Stypocaulon (Swingle, '97), Fucus (Strasburger, '97), and Dic- 

 tyota (Mottier, 1900). 



Swingle has followed the development of the cell-plate in great 

 detail in the apical cell of Stypocaulon. Here each division of the 

 nucleus is followed by a cell-division. The bulk of the cytoplasm 

 presents a very beautiful and typical alveolar structure, and the first 

 indication of a cell-plate is seen in certain alveolae, which show a 

 tendency to arrange themselves across the cell in a transverse plane 

 (Fig. 16, B). As soon as this orientation of the alveolae becomes more 

 marked, the transverse alveolar lamellae form a more continuous plane 

 which, in section, appears as a very fine line. During these changes 

 neither an increase in the number of connecting fibers between the 

 nuclei nor any perceptible change whatever in the arrangement of the 

 kinoplasm was to be seen. Only a few fibers or lines of force, indi- 

 cated by the arrangement of the alveolce of the frothy plasma, extend 

 from the nucleus of the apical cell to the seat of cell-plate formation, 

 and fewer still from the lower nucleus to the same place. It is certain 

 that if there be real fibers, they must be extremely delicate and not 

 numerous enough to lead one to suppose that the cell-plate is laid down 

 by any such process as in the higher plants. 



The author has found that the development of the plasma membrane 

 in the tetraspore mother-cell of Dictyota (Mottier, 1900) is similar to 

 that of Stypocaulon. Here there is absolutely no visible trace of 



