OF EXPERIMENTAL WORK 229 



Tension at Surface. — 0.2 dyne/cm. (E. N. Harvey, 1931 c, 1937; Norris, 1939), when 

 stretched; 0.08 dyne/cm. (Cole, 1932); 0.09 dyne/cm. (Sichel and Burton, 1936). 

 Tension of unfertilized and fertilized eggs, without fertilization membranes, (until 

 just before cleavage) is the same (Cole and Michaelis (1932). See Tension at the 

 Surface, and E. N. Harvey (1954). 



Electrical Properties. — Of two membranes together (McClendon, 1910 b; R. S. 

 Lillie, 1911b, 1916b; Heilbrunn, 1923, 1926b, 1928, p. 183). Membrane resistance 

 is > 100 ohms/cm^ (Cole, 1941 ; see also 1940 and Cole and Curtis, 1938). Capacity 

 about one microfarad /cm^ (Cole, 1941). Zeta potential without jelly, about 30.3 

 millivolts (Dan, 1933). See Electrical Properties. 



Oil Coalescence. — (Kopac and Chambers, 1938; Kopac, 1940a; Chambers, 1944). 



Removal. — By (i) micromanipulation (Kite, 191 2; Chambers, 1921a, 1930, 1938b, 

 1942; Chambers and Kopac, 1937a; Kopac and Chambers, 1938). (2) Washing in 

 isosmotic NaCl or KCl when membrane is being lifted off to form the fertilization 

 membrane (transitional membrane), 1.5 minutes after insemination (Chambers, 

 1942, 1944; Kopac, 1940a). (3) Urea (i M) (Chambers, 1940, 1942; Kopac, 

 1940 a, 1943; Moser, 1940). Pioneer work on development without fertilization mem- 

 brane by action of urea on sirface of egg ( Strongylocentrotus purpuratus) was done by 

 A. R. Moore (1929, 1930 a). He used a molar solution of urea at pH 7 for two 

 minutes, then fertilized the eggs in sea water. Moser (1940) and others (Kopac, 

 1940a) have used a similar technique for Arbacia. (4) Trypsin, used for A. punctulata 

 by Runnstrom in 1950 and by Dan and Mazia in 1951. Dan used (1951 unpub.) 

 o. I % non-crystalline trypsin of Merck in sea water for a few minutes or 4 mg. 

 crystalline trypsin in 100 cc. sea water for 10 minutes; this dissolves the vitelline 

 membrane, and is probably the best way of obtaining fertilized eggs without fertil- 

 ization membranes. Trypsin had been used previously for Psammechinus miliaris and 

 Echinocardium cordatum (Runnstrom, Monn^, and Broman, 1944). In 1932 (unpub.) 

 Kunitz had found that trypsin had an effect on the precursor of the fertilization 

 membrane in Arbacia, preventing its appearance (See A. R. Moore, 1949 a, footnote, 

 p. 243; 1949 b, footnote, p. 207). Best medium for keeping denuded eggs is mixture 

 of NaCl and KCl in proportion of 19 to i in concentrations isotonic with sea water 

 and pH 7.0 (Chambers, 1940, 1944). 



Effect of Calcium. — Brittleness increased (Heilbrunn, 1928, p. 149; Chambers, 

 1944. I949> 1950; see Kopac, 1940 a). 



X-Rays. — (Kopac, 1940c, 1941b). 



Anaesthetics. — (R. S. Lillie, 1914b). 



Ageing. — (Goldforb, 1 937) . 



Centrifugal Force on Breaking. — Break more readily when vitelline membrane is 

 removed (Solano and Mazia, 1953). 



Copper. — Present and absorbed (Glaser, 1923). 



Ribonucleic Acid. — Present (Lansing and Rosenthal, 1949). 



Strength. — Vitelline membrane of different species differs in strength; that oi Arbacia 

 is weaker than that oi Echinarachnius and Asterias (Kopac, 1940a). 



Other Species (additional) and General References 



Carter, 1 924. Sphaerechinus granularis. 



Chase, 1935. Strongylocentrotus purpuratus, Dendraster excentricus. 



Hobson, 1932 b. Psammechinus miliaris. 



Hultin, 1948a, b. Ps. miliaris, Sphaerechinus granularis, Paracentrotus lividus, A. lixula ; trypsin 



and urea, cross fertilization. 

 Hyman, 1923. S./ranciscanus, S. purpuratus, and review of earlier work. 

 Just, 1939 b. The Biology of the Cell Surface, general. 

 Moore, A. R., 1930b. Dendraster. 



