114 E. G. SPAULDING. 



viz., local chemical differences, membranes within the cell, diffi- 

 cult diffusibility of colloidal particles, splitting of molecules, 

 localized processes, resulting electrical changes, make the con- 

 stricted form of the egg at cleavage with its rearrangement and 

 average decrease of tension not only intelligible, but a priori 

 probable. 



//. Means for Changing the Osmotic Pressure Physically. 

 Purely physical changes in the repelling forces of the cell are 

 possible, and the theoretical methods to be inferred from these 

 are confirmed by a number of practical methods for producing 

 artificial parthenogenesis. 



() Hypertonic Solutions. 



The first of these both theoretically and also practically pos- 

 sible methods is the direct raising of the pressure within the cell 

 by first surrounding it with a hypertonic solution and then sub- 

 sequently transferring it to the sea water whereupon segmenta- 

 tion takes place. This method is used for Arbacia, Astcrias, 

 Cluctoptenis, Nereis and Podarkc, and is perhaps the most impor- 

 tant of the artificial parthenogenetic means. Its effectiveness 

 may be explained as follows : In a hypertonic solution there is 

 at first an uncompensated potential difference between the pres- 

 sure within and without the egg, which is equalized by the pass- 

 ing of water through the membrane of the egg to the medium. 

 The relative number of molecules of solute within the egg is 

 accordingly increased. At the same time with a decrease in the 

 egg's radius of curvature accompanying the decrease in size the 

 surface tension has been increasing to compensate the increased 

 internal pressure, though this compensation never quite takes 

 place as long as water continues to be withdrawn, but theoretic- 

 ally is always somewhat behind. The egg thus receives energy 

 from without. With the transferral of the egg to sea water, 

 which is of lower pressure than it is, a potential difference in the 

 opposite direction next exists which is uncompensated by the 

 tension owing to the permeability of the membrane to water ; 

 accordingly an event, viz., absorption of water, takes place until 

 the pressure both within and without is the same. In this proc- 



