No. 466.] STUDIES ON PLANT CELL. — Vill. 725 
appearing as chromosomes during mitosis and the nucleoli. 
These structures are so easily recognized and play such impor- 
tant parts in the events of nuclear division that they command 
attention at once as the essential elements in the nucleus. The 
nucleus may also contain other material such as linin which, 
however, does not seem to have a fixed form or behavior in the 
cell. Finally there are certain kinoplasmic structures, as cen- 
trosomes, centrospheres, and blepharoplasts, whose behavior 
throughout cell history has been much discussed. We shall now 
consider the most important of these structures, those which 
seem essential to the cell in ontogeny. 
The outer plasma membrane naturally retains its morphologi- 
cal entity throughout all cell divisions with such slight changes 
as when new parts are intercalated into its area through the 
vacuoles that are utilized in the segmentation of protoplasm. 
Vacuolar membranes are constantly shifting and cannot be fol- 
lowed during cell division excepting in such cells as have one 
large central vacuole (the tonoplast of De Vries). Sucha cen- 
tral vacuole is much more characteristic of old cells and tissues 
than of young or embryonic regions. There is certainly no 
reason to suppose that it has organic existence through any very 
extended period of the life history. The nuclear membrane 
becomes lost during the prophase of mitosis and there is much 
evidence that its kinoplasm contributes in some cases to the 
formation of spindle fibers. Thus the nuclear membrane disap- 
pears as a structure in the cell during mitosis and new vacuoles 
are formed around the assemblages of daughter chromosomes 
during telophase, leading of course to the formation of fresh 
nuclear membranes at their surface of contact with the sur- 
rounding cytoplasm. 
There is perhaps no region of the cell protoplast that presents 
such different appearances through long periods of the cell his- 
tory as the trophoplasm. This is largely due to the varying 
character of the inclusions which are not in themselves proto- 
plasmic but which give a mixed structure to the trophoplasmic 
regions of the cell. The inclusions may be carbohydrate or pro- 
teid bodies held within spaces in the trophoplasmic groundwork 
or they may be globules of oil or fatty substances. These 
