ADULT COLONY. 43 



or dark line of calcification. Writers on fossil corals particularly have devoted 

 their attention to the subject. For a complete account the reader is referred 

 to various sections of Aliss Ogilvie's work. By some authors the median 

 dark laj'er has been interpreted as 2i primary septiivi^ upon the faces of which 

 the lighter layers, then termed stereoplasm^ have been deposited, the two 

 formations making up the septum proper. Miss Ogilvie considers that the 

 dark appearance of the centers results from the presence of the carbonized 

 residue of the originally unchanged parts of the calicoblasts within which 

 she considers the madreporarian skeleton to be formed. It must be stated, 

 however, that the dark appearance is only seen when sections are viewed by 

 transmitted light. With reflected light the middle region appears lighter 

 than the rest of the septum, and thus can scarcely be occupied by black 

 organic matter. 



Undoubtedly the middle line of the septum is often of a different struc- 

 tural nature from the two sides, and varies much in character in different 

 species. In Paleozoic corals I have occasionally found the middle part to be 

 dissolved away, making the individual septum appear as if formed of two 

 distinct lamellae, more or less completely separated. Furthermore, in the 

 very young septa represented on plate 11, fig. 70, the center of each trabecula 

 is clearly seen, under high power, to be devoid of any inorganic matter. In 

 the rather thick section one can focus down each center for some distance, 

 which would certainly not be the case were it occupied by a calcareous 

 deposit. I conceive that the so-called center of calcification is really the 

 organic center or axis around which the skeletal matter is deposited in a 

 radiating or feather-like manner, and that, at an early stage in the living, 

 growing skeleton, the center is occupied by the mesoglcea-like matrix within 

 which it has been shown that the calcareous fibro-crystals are deposited (p. 34). 

 The organic matrix of the centers is probabl}' in all cases impregnated 

 later with calcareous matter, differing in character and crystallographic 

 orientation from the true skeleton (stereoplasm), as shown by its dii^erent 

 appearance in sections and different solubility. Bourne has remarked (1899, 

 p. 539) : " I suspect that the dark ' centers of calcification ' will be found to be 

 the expression of a core of organic filaments, just as the central dark line in 

 the Alc3'onarian spicule is the expression of the central core of threads." 

 Results published since Bourne's paper seem to confirm his conjecture, only 

 the core is probabl}' homogeneous, and later becomes impregnated with inor- 

 ganic matter, probably from solution, not from calicoblastic acti\'ity. 



The centers of calcification in S. radians are distant from one another 

 on an average about 0.125 mm. and the trabeculse show a corresponding 



