484 MEMOIRS OF THE NATIONAL ACADE:MY OF SCIENCES. 



Ill the most actively growing regions of the colony, where the calicoblast layer forms a 

 broad columnar epithelium, the mesogkea appears as a unifoniily thin layer, entirely free from 

 anv connection with the coralluiii (tigs. 8,16); but in other areas, wlien decalcitication has ))cen 

 carried out slowly, the middle layer displays numerous desmoidal processes on its skeletal surface 

 (fio-s. 7, 17). The most pt>rfect forms of desmoidal processes are conical or p^-ramidal, the base 

 projecting outward; usually they are seen in longitudinal section, but occasionally in transverse 

 section. Toward their free edge the processes stain much more deeply than elsewhere, and are 

 very finely striate, the stria' lieing practically parallel and of equal length. The actual edge of 

 th(? desmoidal processes is usually jagged, as if torn from some attachment. 



Elsewhere the mesogheal lamella exhibits smaller, less regular elevations, which are similarly 

 deeply stained and striate. When a process is <'ut through transversely, either radiating strise 

 are presented at all the levels, or a punctate appearance is revealed. Nuclei are scattered atiout 

 the origin of the processes, but are never found witliin it. The ijrocesses are undoubted 

 outgrowths of tiie thin mesoglceal lamella, the actual continuation being readily observed; but 

 the deeplv-staining character toward the free edge indicates that some structural alteration has 

 taken place. 



Madi'i'pora otiers special advantages for a study of the nature and relations of the skeletal 

 matrix, as. owing to the rapid growth taking place at the apex of the branches, the admixture of 

 ground sulistancc and calcareous deposit is here better preser\-ed than in many corals. In apical 

 2>olyps, which have been hardened in chromic acid, and slowly decalcified by means of weak acetic 

 acid, the organic matrix of the skeleton is obtained hi situ. It presents exactly the appearance 

 of a striated mesoglceal substance, and behaves toward reagents in the same way, and is wholly 

 devoid of nuclei. The fine stritv are arranged in a scale-like manner (fig. 18). The matrix is 

 liest developed at the terminal region of the corallites, and is densest toward the polypal surface, 

 gradually thinning toward what would lie the primary center of calcification within the })ranch. 

 In most preparations it is shrunk from the calicoblast layer. 



Looking at fragments of the corallum under a low magnification (about 60) the surface has 

 the appeai'ance of minute imbricating scales. They are somewhat polygonal in outline, and the 

 free edge is directed obliquely forward, the whole indicating a spiral arrangement, most 

 pronounced in the smaller l)raiiches. Lender a higher magnification, however, each of the 

 apparent scales is seen to ])e but the exposed terminal portion of an obliquely arranged fiber, 

 proceeding from the deeper regions of the corallum, and the superficial scale-like appearance 

 is produced by the overlapping of the numerous fillers. This is also seen in sections of the 

 skeleton starting from the surface, but the drstinction between one fiber and another practically 

 disappears a little lielow the surface. The free edge of each fiber is often slightly jagged, not 

 rounded and smooth, as if torn from some other connection. Viewed by transmitted light, each 

 fiber has an indistinct, delicate, fibrillar appearance, with alternating lighter and darker trans- 

 verse bands, somewhat recalling a striped muscle fiber (fig. 1!))." The matrix left upon decalcifi- 

 cation is thus closely repeated in the microscopic characteristic's of the newly formed skeleton. 



In studving the relations of the corallum to the .soft tissues, sections have been made 

 through the apical region of liranches in which decalcification has but partly proceeded. It is 

 found that the middle jiortion or center of calcification is the first to disappear by the action of 

 the acid, the periphery', where most organic ground substance occurs, being last. 



"Dr. Ogilvie (1896, j). 217) also gives ii figure of the 'enlargeil calcareous fibers oi Madreporti, terminating in 

 what she considers to be calicoblast scales. According to the view there presented, the scales are calcified calico- 

 blasts, but as shown above the polypal tissues afford no support for such a conclusion. The organic matrix referred 

 to by Miss Ogilvie is proved to be a homogeneous, jelly-like substance secreted by the calicoblast layer, within 

 which the calcareous fibrillie are laid down in a scale-like manner. 



