298 E. G. Spaulding 



exerts a pressure from within outward on the cell membrane, thus 

 tending to alter either the volume or the form of the egg, or both; 

 while the other, to be identified with the "tension" of the cell 

 membrane, opposes this. Accordingly, between these two opposed 

 intensities there must be either equilibrium or an uncompensated 

 difference in one direction or the other. Now this condition can 

 be brought about, with the result that the externally directed pres- 

 sure is greater than the internally directed, either by increasing the 

 internal pressure, say, by a chemical splitting, etc., or by decreas- 

 ing the membrane tension, or by doing both at the same time. 

 Conversely, through any means by which the internal pressure 

 is decreased, while at the same time the membrane tension is either 

 kept constant or increased, a potential difference in the opposite 

 direction can be established. 



Omitting further details as to this aspect of the problem, it may 

 be said that the above hypothesis was, in its essential features, pre- 

 sented in the author's paper on the Physics of Segmentation,^ as 

 descriptive and perhaps explanatory of the process of segmenta- 

 tion in general, and in particular of the efficacy of both artificial 

 parthenogenetic and of normal fertilization, as well as of a large 

 number of other experimental results and methods. Its success 

 here, as I venture to deem it, together with its value as a working 

 hypothesis by which there was made clear what the experimental 

 basis for the application of general principles must be, rests as its 

 only justification. 



As against it, I am well aware that the criticism may be offered 

 by, for example, the cytologist and morphologist, that segmentation 

 is too complex for such a simplifying hypothesis to cover it; that 

 it is the result, rather, of a much finer mechanism than is herewith 

 assumed; or it may be objected by the physiologist that there are 

 protoplasmic currents, or a disposition of electric charges, with 

 resultant increased tensions at the equatorial plate, and a decreased 

 tension at the poles, etc. Into the further discussion of these I 

 do not need to enter, for, if my anticipatory statement of them is 

 fair, these objections will have missed the point. For that which 



'Biological Bulletin, vol. vi, 3, Feb., 1904. 



