Microns 



200 



Figure 95. — Early stage of the formation of prismatic layer. Photographed with polarized light. 



precipitation of calcium carbonate. Dotterweich 

 and Elssner (1935) found, however, that calcium 

 carbonate crystals are formed in the extrapallial 

 fluid of Anodonta only in an atmospiiere containinjj; 

 less than 1.5 percent carbon dioxide. In Helix, 

 regeneration of the shell will take place in an 

 atmosphere containing up to 15 percent of carbon 

 dioxide, according to Manigault (1933). Al- 

 though the latter accepted De Waele's theory, his 

 own results seem to prove its inadequacy; and 

 Robertson (1941) remarks that De Waele's hypo- 

 thetical protein compound is without a real 

 chemical basis. Furthermore there are other 



discrepancies in De Waele's results which invali- 

 date his theory. The calcospherites and the 

 protein precipitated from blood and from extra- 

 pallial tluid contained 50 percent organic matter, 

 whereas the new shell contained only 4 percent of 

 it. To reconcile these facts it would be necessary 

 to assume that a great proportion of the organic 

 matter in the new shell must be reabsorbed. The 

 entire process as outlined by De Waelc appears to 

 be higlily improbable. 



Steinhardt (1946) assumed that calcification of 

 the oyster shell is associated with tlie formation 

 of citrate, probably the tricalcium-citrate 



96 



FISH AND WILDLIFE SERVICE 



