388 STUDIES IN GENERAL PHYSIOLOGY 
refractive, and soon thereafter the center also breaks up into 
such droplets. As soon as the blastoderm divides, these drop- 
lets are found distributed over the entire surface, collected 
especially thickly along the furrow between the two cells. 
Before the next cleavage occurs, these droplets again arrange 
themselves in a line which corresponds to the next furrow (Fig. 
103,a). In the process of segmentation part of these droplets 
disappear. This fact, in conjunction with a series of other facts, 
with which we shall become acquainted in the next section of 
this paper, leads me to suspect that this strongly refractive 
substance serves for the formation of the membrane of the 
cleavage-cells of the Ctenolabrus egg.’ That such a membrane, 
or at least a solid surface layer, covers the cleavage-cells of 
the Ctenolabrus egg immediately after a cleavage is completed 
I have observed directly; for folds are often formed on the 
surface of the cells, which are especially distinct immediately 
after a cell-division in the furrow (Figs. 104 and 105, f). On 
the assumption of the existence of a membrane our observa- 
tions can be expressed in a simple way. In the absence of 
oxygen the membranes of the cleavage-cells are liquefied and 
this brings about the fusion of the latter. The material of 
which the cell-walls were formed flows together in droplets 
which coalesce into larger drops in the center of the germ- 
disk. This liquefaction of the material of which the mem- 
brane is formed also renders cell-division impossible in the 
case of lack of oxygen. The assumption of the existence of 
a membrane, or at least of a specific surface film, in animal 
cleavage-cells also brings the mechanics of cell-division in 
animals and in plants into better harmony. 
The fact that in the process of cleavage the droplets 
always collect along the plane in which cleavage is to occur 
later is, as I would suggest in passing, a corroboration of 
1It is now generally assumed that the surface film of cells is formed by lipoids. 
The optical appearance of the droplets mentioned in the text is indeed that of a 
fatty substances. [1903] 
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