The Energy of Segmentation 3^3 



With the pressure constant during a change in volume, this be- 

 comes 



W = p (vo —v^) 



or with the volume constant and the pressure changing, 



W=v (p,-p,) 



Furthermore, in this connection it must be known whether, dur- 

 ing the change, the temperature remains constant or not, t. e.y 

 whether the event be isothermal, or adiabatic. 



That that event of segmentation which we are considering, 

 must, under its normal conditions be isothermal, is, I think, a 

 safe assumption and one universally agreed to; it is in accordance 

 both with that which would seem necessarily true, namely, that the 

 temperature of the organism must, in the large amount of sea- 

 water by which it is surrounded, remain essentially constant, and 

 with what is known of the temperature of organisms in general. 



Do, however, the surface and volume remain constant ? To 

 decide this question, first, measurements of the diameters of a 

 number of eggs in the one-cell stage were made with the ocular 

 micrometer; in doing this as perfectly spherical and as typical 

 individuals as possible were selected; then, likewise, the two axes 

 of each cell in the two-cell stage, where each cell is an oblate sphe- 

 roid, were measured. This was not done for the later stages, owing 

 to the many difficulties which would have been encountered in 

 selecting axes and getting formulae for computative purposes. 

 Of these numerical results averages were taken, and, by the use 

 of the appropriate formulae, the surface and the volume were com- 

 puted. This gave the result, that, while the surface undergoes a 

 marked increase, as accompanying each segmentation, the volume 

 has remained constant. 



Since, then, as accompanying each cleavage, a change of pres- 

 sure is demonstrated by the compensation method, and there is 

 no change in volume as shown above, and the event is isothermal, 

 the equation 



^ 



= E^— £2= — \ V d p becomes here E = v {p^ — p^ 



