568 SYDNEY J. HICKSON. 



Even in exceedingly young platforms small white thickened 

 spots may be seen on the upper surface^ and these arc young 

 buds. The first sign of a young bud is a proliferation of endo- 

 dermal cells on the upper side of the cavity of one of the canals 

 of the platform ; this is followed by an invagination of the ecto- 

 derm above it, which soon takes the form of a wide bag with a 

 narrow mouth. Around this bag eight lobate folds of the canal 

 with its thickened endoderm grow up, the thin laminae of me- 

 soderm remaining as the eight mesenteries. Subsequently a 

 communication is established between the ectodermic invagi- 

 nation and the canal, but I liave been unable to trace the 

 growth of the bud further. 



Formation of the Tabulae. — As Stewart (24) suggested 

 some years ago, the tabulae are formed by a shrinking of the 

 endoderm and its accompanying lamina of mesoderm away from 

 the calcareous wall of the corallite, and the reformaiion of 

 spicules upon it (fig. 10). As the corallite increases in length 

 and the polype recedes farther and fartlier away from the lower 

 parts of the tube the calcareous wall becomes thicker and 

 thicker. This increase in thickness of the wall of the corallite 

 is accompanied probably by a certain loss of vitality of the 

 mesoderm, and this causes the thin strands (fig. 12) connecting 

 the ectodermic and endodermic mesodermic laminae to break, 

 and consequently the endoderm and endodermic lamina of meso- 

 derm shrink towards the axis of the tube. Having shrunk, 

 the mesoderm forms a fresh layer of spicules, which, uniting 

 together, form the tabulae of the dried coral. Having under- 

 gone one process of shrinking, it is quite possible for it to un- 

 dergo a second and to form a second deposit of spicules ; in this 

 manner the condition in which two axial tubes are found may 

 be accounted for. At the nodes the endoderm runs out in the 

 form of canals into the platforms, and these canals, when the 

 slirinking occurs, would have a certain restraining action, and 

 hence the bulging of the axial tubules at the nodes as described 

 above (fig 3), and the formation of the delicate radial tubules 

 figured in woodcut, fig. 1, and in figs. 3, 4, and 5. 



