5°2 



HERRICK E. WILSON 



reticulated structure (Fig. 3, No. 4) when sectioned in any direc- 

 tion. The co-ordination of deposition is such that each plate acts 

 optically and mineralogically as a single crystal of calcite, without 

 other planes of weakness than the cleavage planes developed by 

 crystallization. The margins of the plates are at first rough with 

 sprouting spicule branches (Fig. 2, No. 2, R), but later, upon 

 coming into mutual contact, become smooth (Fig. 2, No. 5). 

 Growth by branching and anastomosis gives way to interstitial 

 growth, 1 and the increase in size is more gradual. Each plate, as 



3 



fy^P^k 



Fig. 3. — Stereom formation: 1, formation of the triradiate spicule by the fusion 

 of seven calciferous cells; 2, basal from a larva of Antedon on the sixth day; 3, basal 

 on the tenth day; 4, ideal representation of regular, reticulate stereom. (1, after 

 Theel; 2, 3, after Bury; 4, after P. H. Carpenter.) 



it now enlarges, is carried relatively outward and away from the 

 adjacent plates, and in the basal cycle not only away from the 

 adjacent plates, but also away from the axial canal, as is shown 

 by growth lines whenever present. When growth of the plates 

 is symmetrical, each plate in a cycle is the equivalent in size and 

 shape of every other plate in that cycle, and has the same angles 

 with reference to the central axis of the cup as the other plates in 

 the cycle. 



b) Reduction of parts by absorption. — This also is a function of 

 ameboid cells, which are similar in appearance to the calciferous 



' Ref. 3S) p. 538. 



