CELL-DIVISION. 



43 



interrupted at these places, but eventually and gradually it attains a 

 uniform thickness. Very soon the cell-plate is a uniform plane, 

 appearing in section as a rather smooth line. 



The cell-plate is not always laid down everywhere simultaneously, 

 but sometimes it appears at first more marked at the periphery. This 

 seems to depend upon the position of the nuclei. It is evident that in 

 Dictyota no differentiated kinoplasmic connecting fibers can be recog- 

 nized by which the cell-plates are formed. It seems that the appar- 

 ently undifferentiated framework of the cytoplasm, consisting of large 

 and small meshes in the immediate region of the cell-plate, is con- 

 verted into a plasma membrane. The cell-plates are certainly formed 

 under the influence of the nuclei, and kinoplasm in some form enters 

 into the process. 



The behavior of the cell-plate toward certain stains, particularly 

 gentian violet, and the character and behavior of the cytoplasm in that 

 region, immediately preceding the appearance of the plasma membrane, 

 strongly suggests that the latter is not an actual transformation of the 

 alveolar walls, but that the substance of the cell-plate is deposited by 

 kinoplasm present in the framework of the cytoplasm. The form in 

 which this kinoplasm occurs here is difficult to determine, but it mat- 

 ters very little whether it takes on the form of a fibrous network or 

 of alveolae, or whether it is present merely as a homogeneous fluid. 



Of the several types of cell formation briefly described -in the fore- 

 going pages, the first, or that which is typical for higher plants, 

 occurs generally in all plants from the liverworts up. It obtains also 

 in Chara and Nitella and has been found by Fairchild ('97) in Basi- 

 diobolus. This method doubtless occurs in other algae and fungi. 



The process of typical free cell-formation, as found in the ascus of 

 the Ascomycetes mentioned, is, so far as known, restricted to this 

 group of fungi 



A process of free cell-formation has been described by Strasburger 

 in the egg-cell of Ephedra, but there it differs considerably from that 

 in the ascus, since centrosomes or centrospheres are not present and 

 the kinoplasmic fibers radiate in all directions from each nucleus. 



The process of cleavage is the method of cell-formation in the plas- 

 modium of Myxomycetes and in certain Phycomycetes. It is also of 

 undoubted occurrence in many algae and in other fungi. 



Whether the kind of cell-plate formation described for Stypocaulon 

 and Dictyota occurs outside of the brown alga?, future research must 

 determine. 



The process of constriction characteristic of Cladophora and Spiro- 

 gyra may be looked upon as a kind of cleavage in which the formation 



