Igor] THE MIDDLE LAMELLA 3 
division, but is a substance secreted into a space between the 
daughter cells. 
The history of the cell plate theory has been carefully sum- 
marized by Timberlake (13), and reference may be made to his 
paper for the literature of the subject. The results of recent 
investigations go to show the essential correctness of Treub’s 
view. In 1898, Strasburger (12) announced that the cell plate 
splits to form a plasma membrane for each daughter cell. Dur- 
ing the splitting process, the rod-shaped elements visible in the 
cell plate, which represent the material furnished to the plate by 
the spindle fibers, are pulled out so as to become extremely thin 
in their middle portion. A middle layer then appears between 
the halves of the original layer. Whether or not the thread-like 
portions of the rods persist, either as part of this new layer or 
as protoplasmic connections between the plasma membranes of 
the daughter cells, he did not determine. So the question was 
left open whether the substance of the wall arises entirely by 
Secretion into the space formed by the splitting of the cell plate, 
or whether a portion of the original cell plate takes part in its 
formation; but Strasburger has wholly abandoned his old notion 
of the identity of the middle lamella with the original cell plate. 
Timberlake (13) finds in the cell divisions of the root tip of 
Allium Cepa, before the appearance of the cell plate elements, 
an accumulation of an orange-staining substance in the equatorial 
zone of the spindle. From the staining reactions of this sub- 
stance, he concludes that it is some form of carbohydrate, 
probably destined for the building of the wall between the 
daughter cells. But, while in general the material of this zone 
Stains like the cell wall, it is not stained, as is the wall, by either 
ruthenium red or iron hematoxylin. The cell plate is formed 
in the midst of this zone by the fusing together of. equatorial 
thickenings of the spindle fibers. The carbohydrate material 
disappears with the formation of the cell plate, but very soon 
after the splitting of the plate the young cell wall (middle 
lamella) appears in the cleft. The cell plate begins to split in 
its oldest part, that is, in the central portion of the spindle, so 
