SECT. 5] 



IN ONTOGENESIS 



801 



that the specific gravity of sea water in which Arbacia punctulata eggs 

 had been standing for some time (i hour) was always lower than 

 that of ordinary sea water (i-0229-i-02i9). This led him to ask 

 whether the eggs or the jellies had abstracted any inorganic material 

 from the water, and in fact experiments showed that there was always 

 a deficit of from 2 to 7 tenths of 

 a milligramme per cubic centi- 

 metre of chlorine in the egg- 

 water after an hour when i part 

 of eggs were suspended in from 

 7 to 10 parts of water. The 

 absorption was real, for eggs 

 which had taken up all the 

 chlorine that they would in one 

 vessel took up no more when 



transferred to another; therefore the phenomenon was not due to 

 an interference with the silver nitrate titration on the part of any 

 egg-secretion. Eggs from which the jelHes had been removed did not 

 show this behaviour, and, as histo-chemically the jelly was found to 

 be rich in chlorine, everything pointed to the jelly being responsible 

 (see Fig. 188). 



Most of what we know about the osmotic pressure of the egg- 

 contents in echinoderms has to be deduced from what has been found 

 to happen as regards the permeability of their membranes. McClendon 

 in 191 o studied the electrical conductivity of masses of eggs before 

 and after their fertilisation [Toxopneustes variegatus and Tripneustes 

 esculentus) in a specially devised conductivity vessel. The following 

 typical figures were obtained: 



Fig. 



Conductivity 



Unfertilised 

 Fertilised .. 



0-01182 

 0-01537 



0-01153 

 0-01277 



There was undoubtedly an increase of electrical conductivity at the 

 beginning of development. This might have been due {a) to in- 

 creasing permeability of the egg-membranes, {b) diminution of fatty 

 phase of the ^gg, [c] dissociation of protein-electrolyte complexes. 

 The second of these possibilities was quickly ruled out by centri- 

 fugation experiments, which showed that just as much fat and oily 

 matter was present after fertilisation as before. Cytolysis experiments 

 showed that the first one was the most probable, and that as in 



