SECT. 5] IN ONTOGENESIS 785 



was directly dependent on the osmotic pressure gradient between the 

 inside and the outside of the tgg. In distilled water it was greatest, in 

 0-004 P^^ cent, sodium chloride less, and so on. Natural isotonicity 

 was with 0-05 per cent, sodium chloride; the space was then normal 

 in size. Over a considerable range it acted as an accurate biological 

 osmometer, but ceased to do so when the concentration of salts out- 

 side became very high. During development the permeabiHty of 

 the egg-membranes varied ; shortly after fertilisation, salts were let 

 out, but not in the later stages. The more eggs developed in a given 

 volume of water, the more ions diffused out in the early stages, and 

 the smaller the perivitelline spaces were. As for the egg-jelly, it con- 

 sists of two concentric portions, both of which are very sensitive to 

 the concentration of ions in the external water, as may be judged by 

 their rapidity of sweHing after the eggs are laid (see p. 323). Hykes 

 has since found that in ordinary water frog's eggs develop quicker 

 when freed from their jellies than they do inside them. In distilled 

 water, however, they will not develop except within the jellies. 



Voss's work agreed very well in general with that of McClendon. 

 McClendon parthenogenetically fertihsed the eggs of the leopard 

 frog, Ranapipiens, by electrical stimulation in distilled water, and then 

 estimated the salt content of the water after a definite interval. The 

 results were as follows: 



Chlorine (as c.c. Total ash in 



i/ioo JV) in water mgm. after 



after 7 hours 7 hours 



Unfertilised ... i-2 6-6 



Fertilised 2-05 1 9-6 1 



Stimulated ... i-q8j ^ °' ii-o 



10-3 



The ash apart from the chloride contained sodium, lithium, potas- 

 sium, calcium, magnesium, sulphate and carbonate, but no phos- 

 phates. There was no doubt that the diffusion of salts from the 

 fertilised eggs (whether naturally or artificially) was nearly double 

 that from the unfertilised eggs. McClendon had previously shown 

 that in the case of sea-urchin's eggs a great many parthenogenetic 

 agents caused an increase in permeabiHty of the egg-membranes, and 

 he regarded this as a general phenomenon. He thought that fertilisa- 

 tion, by allowing increased permeability, permitted the eggs to live, 

 while if they were not fertilised they would swell up and die. Experi- 

 ment showed that the mean diameter of unfertilised eggs placed in 

 water for 30 minutes was 1-52 mm., while the mean diameter of 



