216 C. E. Beecher — Symmetrical Cell Development in JFavositidce. 



The radial arrangement of the tubes in a large hemispherical or 

 cylindrical mass tends to make the axes of the corallites diverge. 

 This divergence can be taken up only by an increase in the diameters 

 of the tubes, or by the addition of new calices between the others. 

 The latter mode is called intermural gemmation. In Favosites and 

 allied genera, the maximum size of the corallites is soon reached, 

 and the expansion of the coral is mainly derived from intermural 

 growth. The study of this method of increase, properly begins 

 after one or more rows of calices have been developed aboixt the 

 par-ent cell, and the calices have reached their full dimensions. 



The following description of a symmetrical system of intermural 

 cell multiplication was observed in a hemispherical specimen of 

 Michelinia convexa, D'Orbigny, from the Corniferous limestone of 

 the Falls of the Ohio. It shows very clearly the stages of develop- 

 ment of the interstitial buds, and their modifications. Other corals 

 were examined to the same end, and were found to agree in all 

 essential particulars, whenever their growth was not irregular from 

 their condition of fixation, or from the excessive development or 

 death of a number of the corallites. An exact number of peripheral 

 buds is not necessary to illustrate the general laws of intermural 

 growth. The buds produced from any given cell cannot always 

 agree with symmetrical method here described, on account of the 

 crowding of similar sei'ies from adjacent or neighboring corallites. 

 After eliminating these variations, it was found that the process of 

 intermural gemmation in general is quite uniform, and closely con- 

 forms to that in 3fichelinia convexa. 



Plate XIV, figure 1, represents diagrammatically the top of a coral- 

 lum composed of a central parent cell and six equal peripheral buds, 

 making seven nearly equal calices in the corallum. The upward 

 growth of these corallites and the divergence due to the direction of 

 their axes tend to separate them from the parent cell. In conse- 

 quence of this separation of the corallites, they would naturally 

 assume a cylindrical form, and there would thus appear triangular 

 interspaces between the tangent points of any three adjacent calices. 

 These angles, therefore, afford the only opportunities for the intro- 

 duction of a set of intermural buds, and their initial triangular form 

 is determined by the conditions of growth. The smallest number of 

 buds which can be symmetrically placed and compensate for the 

 divergence of the corallites is three, one from each alternate angle 

 of the hexagon, Plate XIV, figure 2, 



