PHYSICS OF SEGMENTATION. I I 5 



ess, however, both the pressure and the tension of the egg de- 

 crease, as our theory demands, while the volume and surface in- 

 crease until equilibrium is attained between the two energies 

 within and the pressure without. But therewith new potential 

 differences between these and other energies, c. g., chemical, may 

 have been created, by which the series is continued up through 

 the various stages of development. 



However, in this method it is not directly evident why, instead 

 of segmentation taking place, simply the former size of the egg 

 is not recovered when it is returned to sea water. To account 

 for the constriction actually occurring it is again necessary to 

 make the assumption which is nevertheless theoretically justifi- 

 able in analogy to well-known phenomena in chemistry, that in 

 connection with localized chemical differences some of the col- 

 loidal particles are naturally in a " metastable " condition, which 

 is done away with and chemical changes started by the with- 

 drawal of water, and which therefore cannot subsequently be 

 regained. 1 It may also well be in analogy to known instances 

 elsewhere that so-called preferments become active only on the 

 condition of a certain degree of concentration being present and 

 that, becoming active with the concentration here present, they 

 initiate chemical changes which finally cause the constriction of 

 the cleavage form. 



Action of Temperature Changes. 



The second of the class of methods we are here considering, 

 viz., the physical, has to do with variations in temperature. Two 

 possibilities therefore exist, a raising and a lowering of the tem- 

 perature of the medium and so of the egg. 



A raising of the temperature directly increases the pressure, 

 but it may also produce, as has been seen, a molecular splitting 

 either directly or through the mediate action of preferments, 

 and, as before stated, decrease surface tension. Any one or all 

 three of the effects taken together are essential with the forma- 

 tion of a potential difference in the required direction ; the start- 

 ing of development in Arbacia eggs by heating may be ex- 

 plained in this way. 



1 Cf. Ostwald, Vorlesungen liber Naturphilosphie, s. 271 et 353, et seq. 



