575 
Constituents of the Cell. 
That the centrospheres increase by division cannot be 
doubted. In the early stages of spindle-formation but a single 
one is found at each pole ; and later, commonly just after the 
splitting of the chromosomes, in vascular plants, each is 
replaced by a pair. That the division takes place rapidly 
is shown by the fact that it is rare to find any stages in the 
process. But I have observed two cases in sporangia of 
Osmunda regalis which illustrate the mode of division. In 
Fig. 3 is a nucleus in an early stage of karyokinesis, whose 
centrospheres have arrived at the opposite poles and are 
beginning to divide, apparently earlier than usual. The astro- 
sphere has become biscuit-shaped, and each centrosome has 
divided into two, which still remain very near together. In 
Fig. 8 is seen a later stage in the division of a tapetal nucleus 
and centrospheres, showing the constriction of each astrosphere 
into two about the separated centrosomes. It is, of course, 
impossible to assert that the increase of these organs always 
takes place in this way, since our knowledge of them is so 
fragmentary. But it is natural to ask why, if they can be 
formed de novo in the cell and are derivable from other cell- 
constituents, they should ever have acquired this power of 
division. 
My objections to certain views which have been held 
concerning the relations of the centrosomes and the centro- 
spheres 1 to other cell-contents have since received important 
support. In the previous paper (’94) I tried to show the very 
fundamental structural and chemical differences between the 
centrospheres and the nucleoli, and that Karsten’s belief (’94) 
in the derivation of the former from the latter at the beginning 
of karyokinesis in Psilotum was based on his having overlooked 
the centrospheres entirely, although they are not difficult, com- 
paratively speaking, to demonstrate in that plant. Guignard 
(’94) has so fully confirmed this explanation that further 
arguments seem superfluous to demonstrate the essential 
1 As Guignard points out, not all writers on the subject have appreciated the 
important distinction between the two. 
