192 L. HILLIS-COLINVAFX 



utricles, the crystals grow into the spaces (Fig. 58) until eventually the 

 interutricular space is completely Jfilled (Figs 56, 59). The crystals also 

 extend, depending somewhat on species, into the spaces between the 

 central medullary filaments with, in old segments, some secondary 



Fig. 59. H. incrassata. Cross-section of a iilainent from a mature green segment near 

 the base of the thallus showing amyloplast (a), chloroplasts (c), chloroplasts with 

 starch (Cg), and spherical bodies (s). The segment is well calcified, with aragonite 

 crystals filling the spaces of the segment. Stained with lead tartrate. Scale bar 

 is 1 [xm. 



crystal formation around the aragonite needles (Borowitzka and 

 Larkum, 1977). 



This work by Wilbur and his coworkers (1969) demonstrated that 

 the process of calcification in Halimeda occurs in the spaces of the 

 segments, and they pointed out that since the peripheral utricles usually 



