OF EXPERIMENTAL WORK I 79 



vitelline membrane (R. S. Lillie, 191 1 a, 1916b; Kite, 1912; Heilbrunn, 191 3, 1915a, 

 1928, p. 259; Glaser, 191 3, 1915, 1924; F. R. Lillie, 1914; Chambers, 1921a, 1924; 

 et al.). But some thought it arose de novo, by secretion of a membrane substance 

 (E. N. Harvey, 1909, 1910 b, 19 14) or by precipitation of oppositely charged col- 

 loids (McClendon, 1909b, 191 1, 1914a; see Garrey, 1919). 



More recently it has been definitely shown that the fertilization membrane comes 

 from the vitelline membrane, since it does not form when the vitelline membrane 

 has been removed by KCl, urea, or trypsin. (See below under "Removable") ; 

 development takes place without a fertilization membrane. (See under Vitelline 

 Membrane). It has also been shown that the cortical granules which disappear on 

 fertilization help in the formation of the fertilization membrane (Moser, 1939a; 

 etc.). See under Cortical Layer. It is now generally accepted that the fertilization 

 membrane is the pre-existing vitelline membrane whose properties have changed, 

 together with cortical granule material, in Arbacia and in other species (See Runn- 

 strom, 1952a, Chapt. VII "The Origin of the Fertilization Membrane"). 



Formation. — The fertilization membrane starts to form at the sperm entry in about 

 20 seconds after it touches the surface (E. B. H.; see also Just, 1928 a, 1939 b, p. 105; 

 Moser, 1939a; et al.). See Part II, Fertilization. This was first observed in Asterias 

 glacialis by Fol (1877). It is fully elevated about two minutes after fertilization at 

 23 °C. (E. B. H.). 



According to Heilbrunn (191 3, 1915a, 1924a), membrane elevation is due to a 

 lowering of the surface tension, since all parthenogenetic agents do lower the sur- 

 face tension. According to Loeb (1913 a, p. 212; 1912 a, p. 136, 150), it is due to 

 swelling of a colloid and liquefaction of the surface. According to E. N. Harvey 

 (1910 b) and R. S. Lillie (1911a) it results from increase in permeability. Jelly is not 

 necessary for its formation (E. N. Harvey, 19 14; F. R. Lillie, 19 14; F. R. Lillie and 

 Just, 1924, footnote p. 453; et al.). Oxygen necessary for formation of fertilization 

 membrane in fertilized eggs, because it is necessary for motility of sperm (E. B. 

 Harvey, 1930; Barron, 1932). Not necessary for membrane formation in partheno- 

 genetic eggs (Loeb, 1913a, p. 215; Kitching and Moser, 1940). 



Structure. — No regular structure or pattern is shown by the electron microscope 

 (E. B. Harvey and Anderson, 1943). This is also true of Pj. miliaris, according to 

 Mitchison (1953). Hillier, Lansing, and Rosenthal (1952) say that the membrane, 

 in Arbacia, is composed of a single layer of loosely packed particles. 



Thickness. — Though readily visible, its thickness is not measureable with a light 

 microscope. Measured with an electron microscope, it is 250 A when first elevated 

 and dried (E. B. Harvey and Anderson, 1943). According to Hillier, Lansing, and 

 Rosenthal (1952), it is less than 300 A thick. A recent measurement with the elec- 

 tron microscope, of the fertilization membrane of a different species, Ps. miliaris, 

 gives its thickness as 100 A (Mitchison, 1953). The dry thickness as measured with 

 an interference microscope is given as about 160 A (Mitchison and Swann, 1953). 

 The early membrane is easily ruptured (R. S. Lillie, 1916b; E. B. Harvey, 1933b, 

 et al.). It becomes thicker and tougher after about five minutes (Heilbrunn, 1915a; 

 Chambers, 1921a; E. B. Harvey, 1933b; E. B. Harvey and Anderson, 1943; et al.). 

 In Ps. miliaris its thickness is given as about i(i. (Runnstrom, Monne, and Wicklund, 

 1946). 



Specific Gravity. — Lighter than the eggs. If placed in distilled water immediately 

 after they are formed, the membranes can be freed of the egg material and form a 

 layer above the eggs (E. B. Harvey and Anderson, 1943). Whitaker (1933a) found 

 that sometimes they were thrown off in the centrifuge and then formed a layer above 

 the eggs. 



Elasticity. — Can stretch with centrifugal force from 82 \l diameter (normal) to 

 140 (X when first formed; they resist stretching after five minutes (E. B. Harvey, 

 1933 b; E. B. Harvey and Anderson, 1943). Expansibility (Chambers, 1942). 



