POSTLARVAL DEVELOPMENT. 113 



When any of the young polyps, two or three weeks old, were macerated 

 later, it was found that a thin continuous plate remained adherent to the 

 incrusted surface ; but none of the earliest stages in its formation were 

 obtained, showing whether it originated as a continuous layer or in sepa- 

 rate parts. All the poly pal tissues have been macerated from the young 

 coralla represented on plate 4, figs. 19 and 20 ; they now show a well-developed 

 basal plate, slightly upturned at the periphery, and bearing the septa on the 

 upper surface. 



A portion of the basal plate of a macerated corallum of two weeks, magni- 

 fied about 300 times, is represented in surface view on plate 1 1, fig. 69. At the 

 upper boundary of the figure the epitheca also is shown in section, and upon 

 the plate are the first thickenings which will form the septa. The entire 

 surface of the plate exhibits a number of very thin, flat scales, roughly polyg- 

 onal in form. Sometimes they appear as if joined edge to edge like the 

 cells of an endothelium, or at other places as if overlapping. The average 

 diameter of the individual scales is about 0.03 mm. In most of them a distinct 

 fibrous structure can be recognized, the fibro-crystals either lying parallel 

 or, less often, presenting a radiating appearance. There is, however, no 

 suggestion of the fibers being arranged around a center of calcification, as in 

 the trabeculae of the septa. 



According to Miss Ogilvie (1897, pp. 114-117) similar scale-like elements 

 are present on the surface of the corallum of corals generally, and she has 

 succeeded in isolating them from the almost transparent dissepiments of 

 Galaxea. Their breadth in this genus varies on an average from r ,01 to 

 0.015 mm., while their height is about 0.003 mm - A number of fibers are 

 present in each, sometimes in the form of divergent groups, but often lying 

 loosely side by side. It was from a study of these that Miss Ogilvie came 

 to the conclusion that each isolated skeletal element represented a calcified 

 calicoblast cell, thus returning to the old view of von Heider as to the origin 

 of the calcareous skeleton of corals in contrast with the more accurate results 

 of von Koch, Bourne, and Fowler, which show that the calcareous matter is 

 secreted wholly external to the ectodermal cells. The structure of the calico- 

 blast layer in the polyps of Siderastrea is also very conclusive as to the 

 ectoplastic origin of the calcareous fibers. Everywhere it has been found to 

 be a simple layer, never many cells deep, as would be the case were the 

 calicoblasts themselves calcified and shed from time to time to build up the 

 skeleton. 



From the relationships of the basal plate to the polyp, increase in its thick- 

 ness can obviously take place only on the^upper surface. In addition to the 



