792 



BIOPHYSICAL PHENOMENA [pt. iii 



o Daphnia pulex(embryo) 



X .. .. (perivitelline fluid) 



® Daphnia magnaCembryo) 



+ ■. " (perivitelline fluid) 



differentiation proceeds normally in spite of the suppressed growth, 

 so that well-formed dwarf embryos are produced. These will never 

 hatch if the osmotic pressure of the exterior remains high, and will 

 die within the membranes. The necessary minimum osmotic pressure 

 for "closed development" is that of JV/30 NaCl. Embryos artificially 

 Hberated from their membranes will continue to develop normally 

 in either pond water or salt solution of the same strength as that 

 which prevents hatching, or even in stronger salt solutions. The volume 

 of the finished embryo in the cladocerans is at least three times as 

 great as that of the egg before the bursting of the egg-membrane, 

 and as this increase in volume can be completely inhibited by raising 

 the osmotic pressure of the ex- ^ ^o^ 

 ternal hquid, there can be no 

 doubt that the cladoceran egg is 

 arranged to absorb a great deal 

 of water from its environment. "^1^ 

 This property is of much interest 

 and will be referred to again 

 in the succeeding section (see 

 p. 896). In distilled water the 

 preponderance of the inner over 

 the outer pressure at hatching 

 would be about A — 0-75° but 

 in pond water a little less, as the latter has itself an osmotic pressure 

 of A — 0-02°. At external osmotic pressures of about A — 0-176°, 

 50 per cent, of the eggs go into "closed development", which shows 

 that half of them can, if necessary, raise their internal osmotic pressure 

 to higher than this. But they cannot go further, and though they may 

 overcome JV/20 NaCl they will not manage JV/15 NaCl. Ramult's dis- 

 covery of " osmotic hatching" in cladoceran eggs is paralleled by the 

 fact that phyllopod eggs (e.g. Artemia salina) will not hatch in the 

 strong salt solutions in which the adults normally live (Becking) . This 

 is a remarkable instance of an experiment done for us by Nature. 



The eggs of other arthropods have been little investigated from 

 this point of view. Walther in 191 3 placed the eggs of the crab 

 Telphusa fluviatilis in water to which magnesium salts were added, and 

 found by microchemical methods that not until many days had 

 elapsed was any magnesium to be found inside the egg-membranes. 

 In other eggs, of course, the salt penetrates much more quickly, 



Days 



10 20 30 



2 

 Fig. 



