SECT. 5] 



IN ONTOGENESIS 



791 



0) 300 

 > 



200 



with fertilised eggs oi Daphnia pulex and Daphnia magna. All his results 

 are shown in Fig. 183, from which it will be seen that the osmotic 

 pressure of the perivitelline liquid rises pari passu with that of the 

 embryo, but at a rather lower level. 



The cladoceran embryo, hke that of the silkworm, passes through 

 a hibernation period when part of its development has been com- 

 pleted. Przylecki found that 

 during this time the osmotic 

 pressure remained at the final 

 level reached after the main rise, 

 i.e. from -- 0-734° to — 0-74°, 

 but after two months there was 

 a lowering of 30 per cent, or so, 

 followed by a rise until hatching. 

 Przylecki estimated that of this 

 lowering about 24 per cent, was 

 caused by absorption of water, 

 and 34 per cent, by excretion 

 of osmotically active substances 

 into the perivitelline fluid, as in 

 the early stages of the frog's egg, 

 while the remainder he could not 

 account for. Przylecki gave no 

 figures for the osmotic pressure 

 of the unfertilised Daphjiia egg, 

 but presumably it was somewhere about — 0-7°, in which case the 

 events in amphibians are closely paralleled by those in cladocerans 

 (see Fig. 185). 



Osmotic pressure seems to play a very important part in the normal 

 embryonic life of cladocerans. Ramult, working on Daphnia pulex, 

 found that if these animals, soon after the eggs had been laid in the 

 brood-pouch, were transferred from pond water to sodium chloride 

 solutions the embryos did not hatch at the proper time. During 

 normal embryonic life, two membranesjbuj^t^ first the egg-mem- 

 brane, and then the larval membrane. Although differentiation 

 goes on continuously, growth does not, but has periods of slowness 

 while waiting for the membranes to burst, the larval one being 

 more extensible than that of the egg. In "closed development", 

 artificially induced by a rise in osmotic pressure of the environment, 



30 60 



Hoars after Fertiliza,tion 



Fig. 184. 



