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 f act , 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. 1 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 011 the 

 surface of the cells, which are especially distinct immediately 

 after a cell-division in the furrow (Figs. 104 and 105, /). 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 



1 It 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 substancj. [1903J 



