Figure 93.— Small 



30 

 Microns 



granules 

 en 



es (calcospherites) in conchiolin shortly after its secretion by the mantle. Black and w 

 ilargement of a Kodachrome photograph taken with polarized light. 



■hite 



by later deposition of conciiiolin, the secretion of 

 which continues during calcification. The details 

 of this prosess have not yet been described. 



THEORIES OF CALCIFICATION 



Studies of shell calcification fall into two major 

 categories. One type of work places the emphasis 

 on the identification of calcium-secreting cells or 

 organs; the other approaciies the problem from 

 the biochemical point of view. It has been 



generally accepted that calcium carbonate, sepa- 

 rated from blood, is secreted as colloidal gel by 

 certain cells at the edge of the mantle and that 

 crystallization takes place outside the cells 

 (Crofts, 1929;Dakin, 1912;Kuyper, 1938) between 

 the conchiolin sheet and the mantle. Separation 

 of calcium is not, however, confiiied to the surface 

 cells of the mantle. The calcium-secreting cells 

 may be subepithelial, as in Patella (Davis and 

 Fleure, 1903). In the calcification of the epi- 



94 



FISH AND WILDLIFE SERVICE 



