\ 



1° 



J L 



Microns 



200 



Figure 94. — Calcite crj'stals of new o.vster shell about 24 to 36 hours after its formation. Black and white enlargement 



of a Kodachrome photograph taken with polarized light. 



phragm of Helix pomaha (Prenant, 1924, 1928), 

 the calcium is liberated by the leucocytes in the 

 connective tissue of the mantle. In the case of 

 pearl formation, Boutan (1923) has shown that 

 calcareous deposits are formed by amoeboid cells 

 which crawl through the mantle epithelium, while 

 the latter secretes the concentric layers of the 

 organic matrix (conchiolin). 



De Waele (1929) approaciied tlie calcification 

 problem from the phj-siochemical point of view. 



Working wnth Anodonta cygnea he has shown that 

 tiie e.xtrapallial fluid between the mantle and the 

 shell is chemically identical with blood. Ex- 

 posure of this fluid to air causes the formation of a 

 precipitate, which consists of a suspension of 

 calcium spherules in protein solution. He there- 

 fore assumed the existence in the pallial fluid of a 

 hypothetical compound consisting of protein, 

 carbon dioxide, and calcium carbonate. The 

 escape of carbon dioxide would then cause the 



THE MANTLE 



95 



