362 IMMUNOLOGICAL STUDIES 



entry of this ioii which is nomially in very low concentration in- 

 side the cell. However, no significant increase was found. Deter- 

 minations were also made of the relative tension at the surface 

 of untreated and treated eggs, by the method of separating the 

 egg into light and heavy halves by centrifugal force. An increase, 

 estimated as approximately 40%, was found in the treated eggs. 



Of special interest in this work is the fact that highly effective 

 antisera are produced by immunization with fertilizin, since much 

 is known concerning the chemical and biological properties of 

 this substance (cf. Tyler, 1948, 1949, 1955a,b, 1956b; Runn- 

 strom, 1949, Vasseur, 1948, 1952). The fertilizins of sea urchins 

 are obtainable in a form that is electrophoretically and ultracen- 

 trifugally homogeneous, and whose purity is further evidenced 

 by the effective removal of nitrogen-containing substance upon 

 absorption with homologous sperm. Molecular weight determi- 

 nation give a value of about 280,000 for the fertilizin of Arhacia 

 punctulata, and fertilizins of other species are in the same range 

 as judged from then- sedimentation constants ( Tyler, 1956b ) . The 

 molecule is highly elongate (axial ratio of about 20:1, calculated 

 as a prolate ellipsoid with 0.4 gram of water of hydration per 

 gram), which is consistent with its gel-forming properties. Chem- 

 ically, sea urchin fertilizins belong to that class of sugar- and 

 amino acid-containing substances termed glycoproteins or muco- 

 polysaccharides. In several species of sea m'chins that have been 

 examined there are two kinds of sugars and 14 kinds of amino 

 acids in the molecule, and the content of the two kinds of residues 

 is approximately the same. Knowledge of the composition of the 

 fertilizins should prove useful in investigation of the determinant 

 groups that are involved in the production of the cleavage-block- 

 ing antisera. 



Experiments on the biological properties of fertilizin have led 

 to the view (cf. Tyler, 1955a; Tyler and Metz, 1955) that it repre- 

 sents the specific receptor substance for the union of egg and 

 sperm in fertilization and is largely responsible for the tissue and 

 species specificity of the process. When unfertilized eggs are 

 treated with antisera against fertilizin, they lose their fertilizabil- 

 ity. This, however, does not necessarily mean that the antibodies 



