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J. GRAY 



egg-surface immediately under the vitelline membrane. In 

 the case of the echinoderm c^gg it seems probable that essen- 

 tially the same change takes place. 



Many years ago Loeb (25) showed tliat the fertilization 

 membranes collapsed when placed in sea-water containing 

 albumen, and that on transference to normal sea-water the 

 membrane regained its normal spherical shape. He concluded 



Text-fig. 1. 



Diagram illustrating the origin of the fertilization membrane of 

 Echinus miliaris. A. Unfertilized egg. B. Fertilized egg. 

 ft. Zona pelhxcida containing an electro-positive colloid. 

 h. Vitelline membrane of unfertilized egg. 

 c, Fertilization membrane formed by the interaction of a, 



and an electro- negative colloid, d, which is set free when h is 



emulsified. 



that the extrusion of the membrane was therefore due to the 

 existence of an osmotically active colloid (proteid) within the 

 fertilization membrane. Now the osmotic properties of such 

 a colloid are markedly affected by the presence of hydrogen 

 and hydroxyl ions ; hence, if Loeb's conclusion be correct, 

 the degree to which the fertilization membrane is extruded 

 should be altered by altering the acidity or alkalinity of the 

 sea-water. This is actually the case. If the water be made acid 



