SECT. 5] 



IN ONTOGENESIS 



783 



AD 



-0'450i 



■0-40O 



Thunberg criticised the statement of Backmann & Runnstrom 

 that frog's eggs would not develop in solutions isotonic with the 

 adult blood serum, and cited some not very good work of 

 Overton's in support of the opposite view. But the observations 

 of Backmann and his asso- 

 ciates were later confirmed by 

 Bialascewicz, who found the 

 fall of osmotic pressure on fer- 

 tilisation, the subsequent rise 

 in osmotic pressure and the 

 corresponding rise in water- 

 content. Citing the experi- 

 ments of Siedlecki, who had 

 found the embryos of the 

 Javanese frog {Polypedatus rein- 

 wardtii) incapable of develop- 

 ing in normal pond water if 

 removed a day or two early -0-350- 

 from their envelopes, Bialas- 

 cewicz concluded that the peri- 

 vitelHne liquid was not by any 

 means ordinary water, but 

 contained osmotically active 

 substances which could not 

 diffuse out through the egg- 

 membrane, and so maintained 

 a definite pressure gradient 



0-1 0-2 0-3 0-4 0-5 



Weight of larvsLin gms. 



Fig- 179- 



involving constant tension on the envelopes. The values which 

 Bialascewicz obtained for osmotic pressure are shown in Fig. 179. 

 He also made osmotic pressure determinations on ovarial eggs 

 of other anura {Rana esculenta — 0-446°, Bombinator igneus — 0-445°). 

 For ovarial eggs his A was — 0-444°, ^^^ ^^^ adult serum -- 0-479°, 

 a difference which he emphasised as resembling the corresponding 

 difference in avian eggs. He explained the initial fall of osmotic 

 pressure in the egg itself rather by an excretion of osmotically active 

 substances into the newly formed perivitelline space than by an 

 adsorption on to colloidal particles. Thus the perivitelHne fluid 

 would be a salt solution of definite strength, and he had himself 

 previously shown that the embryos of Salamandra and Molge would 



