182 L. HILLIS-COLINVAUX 



dark cycle but is not necessarily related, noticeable greening occurs. 

 Adhesion of the peripheral utricles also takes place at about this time, 

 and the first granules of calcium carbonate appear. 



The growing medullary filaments are thin-walled (0-1-1 [jim). Their 

 tips show a gradient of organelles, and three regions have been delimited 

 (Borowitzka and Larkum, 1977). At the very apex and extending some- 

 times for a length of about 4 [i,m is a region of numerous small vesicles, 

 some with granular material resembling wall components, others with 

 osmiophilic material. 



The region below the tip is characterized by much endoplasmic 

 reticulum and many mitochondria with DNA-like fibrils in part of 

 their matrix. Some of the mitochondria are several times the length of 

 mitochondria elsewhere in the thallus. There is also much nuclear 

 division. 



The third growing-tip region contains many small vacuoles together 

 with microtubules, mitochondria and young plastids with and without 

 starch depending on the age of the filament. 



Golgi bodies are present throughout the three regions although not 

 uniformly so. Migration of organelles from the preceding segment and 

 their participation in the development of the new segment have not 

 been studied. 



The tips of the filaments forming the cortex show a similar differ- 

 entiation into three regions (Borowitzka and Larkum, 1977). 



The remainder of the filaments of the developing segment may be 

 somewhat more vacuolate, and other organelles may be present such as 

 amyloplasts (Fig. 54). The spherical and electron-dense bodies (Section 

 I) may occur in some material (Wilbur et al., 1969). 



Young peripheral utricles, when their walls become laterally 

 attached, possess many well-developed chloroplasts as well as young 

 plastids, amyloplasts and a vacuole which may contain many spherical 

 bodies (Borowitzka and Larkum, 1977). Calcification begins at about 

 this stage. 



(a) Adhesion of peripheral utricles. In cylindracea the osmiophilic 

 covering lamella bulges outwards, away from the rest of the filament 

 wall, and appears to fuse with the covering lamella of the neighbouring 

 peripheral utricle (Borowitzka and Larkum, 1977). The resulting space 

 between covering lamella and filament wall contains a granular material. 

 This is the only species for which lateral adhesion has been studied 

 and reported at the ultrastructural level. The process is probably similar 

 in most other species, but interesting variations may occur. In some 

 species, particularly macrophysa (Sections III, IV), the peripheral 



