A 



o 



^:iD 





k — ^ 





V 



rr~ 



* I 1 I I 



Microns 



Figure 96. — Photomicrograph of a piece of new shell of Crassostrea virginica taken 28 hours after the beginning of calcifi- 

 cation. Small calcite crystals are randomly distributed, and calcospherites scattered over the entire field of view 

 are in places densely packed between the larger crystals. 



(C6H507)2Ca3 + 4H20. The observation that citric 

 acid is formed in connection with carbo- 

 hydrate metabolism, and that citrate is quaUta- 

 tively precipitated from a solution which also 

 contains phosphate and calcium ions in a suitable 

 concentration (Kuyper, 1938, 1945a, 1945b), 

 forms the basis of his conclusion. The citrate in 

 the precipitate is found not as calcium citrate but 

 in a somewhat more comple.x form in which cal- 

 cium is combined with both phosphoric and citric 

 acids. This is verified by the results of the 

 analyses shown in table 11, in which the oyster 

 shell was presumably 0. eduiis. It is rather 

 difficult to arrive at a definite conclusion regarding 

 the role of citric acid in the calcification of oyster 

 shells, but Steinhardt's observations establish the 

 presence of calcium phosphate in the oyster shell, 

 which was supposed to consist primarily v( car- 

 bonates; and an abundance of calcium phosphate 

 in the mantle was demonstrated by Biederniann 

 (1914). 



During recent years (Bevelander, 1952; Bevel- 

 ander and Benzer, 1948; Bevelander and Martin, 

 1949; Hirata, 1953; Jodrey, 1953) considerable 

 advances in the study of the processes of calcifica- 



THE MANTLE 



Table 11. — Analyses to calcified materials according to 



Steinhardt 



[All figures are in percentages] 



tion have been made. It had been generally 

 assumed that the small granules appearing on the 

 surface of the conchiolin consisted of calcium 

 carbonate, but Bevelander and Benzer found that 

 the}' are made of calcium phosphate. It is not at 

 all clear how the calcium phosphate of the granules 

 is converted into calcium carbonate, which is the 

 final product of calcification in the oyster shell. 

 It is doubtful tliat the conversion is accomplished 

 by threct reaction between the calcium phosphate 

 and tlie carbonate, because such a process would 

 require very higli concentrations of carbonate. 

 Tiie explanation proposed by Bevelander and 

 Benzer implies that calcium phosphate may be 

 dissolved by the action of organic ions which in 

 some manner bind calcium. Phosphatase may 



97 



