Membrane Formation 



Z7^ 



Sea urchin eggs (Arbacia and Toxopneustes) also round up on 

 standing for some time in sea-water thus indicating an increase 

 in surface tension. They also become fertilizable by foreign 

 sperm on standing^^ (ca. 6 hrs.) or by treatment with an acid or 

 alkali (in concentrations too weak to cause membrane formation.) 

 It appears as if the change undergone by the eggs on standing 

 were in the direction of increased permeability and that the egg 

 must start toward development in order to be fertilized by foreign 

 sperm. Some eggs do undergo division on standing but it is 

 probable that accessory factors are responsible for this. 



In order to determine further the nature of the change taking 

 place in mature unfertilized sea-urchin eggs on standing in sea- 

 water, and especially if thischangewerein thedirectionof increased 

 permeability, I tried if any less acid were required to cause mem- 

 brane formation six hours after shedding. At this time the eggs 

 of Toxopneustes become fertilizable by foreign sperm. The fol- 

 lowing table is a typical result: 



July 14, igog. Eggs taken 12.^0 p.m. Temp. 55° to J4°. 



Another experiment like the above gave a similar result, the 

 optimum treatment, both as regards time and concentration, 

 about coinciding three-fourths and 5I hours after taking the eggs. 

 If any, a very slightly longer treatment appears more favorable. 

 Certainly the eggs require no less acid to cause membranes to 

 form after standing for six hours. In the above experiment the 

 per cent of eggs which could be caused to form membranes was 



" See Tennent, Biol. Bull, xv, p. 127, 1908. 



JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 8, NO. 4. 



