4b2 THE AMERICAN NATURALIST. [Vol. XXXVI II. 



Swingle ('97) for Stypocaulon give details of the region of the 

 cytoplasm that forms the partition wall between the daughter 

 nuclei. There is a zone of fine meshed protoplasm between 

 much larger vacuoles. It is possible that some very long fibrillse 

 may connect the daughter nuclei with this zone, but they do not 

 form a cell plate. Consequently the wall must be developed in 

 this delicate alveolar layer, which probably splits along some 

 plane of vacuoles. The process of cleavage is then really related 

 to such activities of vacuoles as occur in the sporangium of the 

 Mucorales and in the Plasmodium. But the position of the alve- 

 olar layer may be determined by the fibrillae, since it is always 

 situated nearest to the smaller of the two daughter nuclei. 



It seems likely that the process of cleavage in the Fucales will 

 be found to be similar to Stypocaulon when the details of struc- 

 ture in the internuclear cytoplasm is known. So this group, with 

 others, is likely to furnish conditions in which spindle fibers may 

 determine the position of the cell wall and exert a directive influ- 

 ence upon it without actually laying down a cell plate. As has 

 been pointed out, the splitting of the cell plate is probably a 

 cleavage along a very thin flat vacuole, so that the process in its 

 essential characters is the same as cleavage through a series of 

 vacuoles. Thus cleavage by the cell plate is possibly an out- 

 growth from that phase of cleavage by constriction in which the 

 extensive fusion of vacuoles determines the planes of separation. 

 The important advance lies in the new factors, introduced through 

 the activities of fibrillae, which become very conspicuous as actual 

 contributors of material to the kinoplasmic film which is laid 

 down as the cell plate. This function of the fibrillas probably 

 developed slowly from conditions such as those in Stypocaulon 

 and Pelvetia, where their influence upon the position of the cell 

 wall, if any at all, can scarcely be more than directive. 



3. Free Cell Formation. 



Whenever a nucleus becomes the center around which cyto- 

 plasm is gathered and separated from the rest of the cell con- 

 tents, so that the new cell lies freely in the protoplasm of the 

 old, this vi, free cell formation. Illustrations are presented by 



