834 BIOPHYSICAL PHENOMENA [pt. iii 



distance, measured optically. In a second paper he studied the change 

 in refractive index during the period from fertilisation to first cleavage 

 in the sea-urchin's egg. A small fall after fertilisation led to a large 

 rise after the appearance of the diaster, and a rapid fall immediately 

 before cleavage, the outside limits of refractive index within which 

 these changes took place being 1-381 to 1-405. Their significance was 

 doubtful. 



The effect of heat on the physical properties of egg-cells has been 

 studied by Achard. The volume, calculated from the diameter, 

 attained a definite maximum at 35°, being 44-5 . io~^ c.c. at 20°, 

 54-3 at 35°, and 46-0 at 41°. The specific gravity measured by im- 

 mersion of the eggs in solutions of equal .osmotic concentration but 

 different density varied little, but had a slight peak at 36°. The 

 electrical charge had a maximum at 35°. The oblateness (calculated 

 from measurements of maximum and minimum diameter) had a 

 maximum at 35°, and the surface tension (calculated from the 

 oblateness) had a minimum at the same temperature. Achard con- 

 cluded that the physical properties of these egg-cells could be inter- 

 fered with to a slight degree apparently without preventing normal 

 fertilisation and cleavage. The curves she obtained are given in 

 Fig. 206, and show how 35° is in nearly all cases a critical point. 

 The important point is that it is also a critical point biologically, for 

 below it the percentage of eggs forming normal fertilisation mem- 

 branes is uniformly 100, and above it falls off, while just the same 

 applies to the percentage of eggs successfully accomplishing their first 

 cleavage. 



Vies himself did a good deal of work on the surface tension of 

 echinoderm eggs, treating them from the point of view of the physics 

 of semiliquid drops, and examining their departure from the form 

 of a perfect sphere under different experimental conditions. The 

 surface forces acting on an egg can be evaluated, according to Vies, 

 from the degree of oblateness or flattening which the egg suffers 

 when it rests on a horizontal surface under the influence of gravity. 

 Unfertilised sea-urchin's eggs, devoid of jelly, show minima of 

 flattening between pH 3 & 5 and 8 & 10. The tensions involved 

 are of the order of 10 to 25 dynes per centimetre. ^ After fertilisation 

 the surface tension is reduced ; it rises again before the fusion of male 



^ A much lower value (1-3 dynes per centimetre) is obtained for Chaetopterus eggs by 

 observing their fragmentation in the microscope-centrifuge (Harvey & Loomis; Harvey). 



