III. 7 



OSMOTIC PRESSURE 



117 



developed as normally in fresh water as in sea-water, only 

 a blastoderm with occasionally a dwarf embryo was formed in 

 a 5 % solution of sodium chloride in sea-water, and segmentation 

 was arrested in the thirty-two-celled stage when the concentra- 

 tion of the salt was raised to 10 %. Older eggs were, however, 

 far less sensitive, and after three or four days the embryos could 

 be placed directly in a 27-5 % solution without arresting their 

 development, though the heart beat more slowly and differentia- 

 tion was less rapid. 



FIG. 60. A and C, formation of ex-ovates in the egg of Arbacia by 

 dilution of the sea-water ; k, nucleus ; m, egg-membrane ; B and D, 

 blastulae formed from A and C; B becomes constricted into two blas- 

 tulae, each of which gives rise to a Pluteus ; D produces a single 

 Pluteus. (After Loeb, from Korschelt and Heider.) 



The eggs can nevertheless be acclimatized to the salt. Re- 

 moved from the 10 % solution after the thirty-two cells had 

 been formed to ordinary sea-water for eighteen hours, they were 

 capable, when once more replaced in the strong solution, of 

 giving rise to embryos which lived for a considerable time. 



Similar experiments made on Arbacia showed that though 

 cell-division is suppressed in the hypertonic solution (2 % sodium 



