32 SIDERASTREA RADIANS. 



outline, often with many adhering particles which stain differently from the 

 layer itself (plate 8, fig. 47). 



Away from the actual growing apices of the skeleton, both downward 

 and peripherally, the skeleton-lining tissues at once begin to undergo a 

 marked alteration ; both ectoderm and endoderm become greatly thickened 

 until they are three or four times their former size (plate 8, figs. 45-48). 

 The thickened endoderm is reticular, with many oval nuclei arranged in a 

 regular zone towards the margin ; Zooxanthellae are also numerous, and dis- 

 tinct mucous accumulations appear. 



Vacuoles extend nearly across the layer, and usually appear clear and 

 perfectly transparent, but when stained with iron hsematoxyliu they present 

 a grayish reticulum. They are evidently mucous iu character, and would 

 seem to represent individual cells. In sections stained with Delafield's 

 haematoxylin and picric acid the vacuolar spaces stand out conspicuously, 

 the boundaries and reticulum being stained a dense blue, as with mucous 

 cells generally, and appearing as if enclosed in a yellow granular matrix. 

 Usually the mucous spaces do not extend as far as the outer margin of the 

 layer, and not always to the mesogloeal boundary. 



The general cytoplasm of the calicoblast ectoderm is finely granular 

 throughout and does not stain readily except in blue-de-Lyon, which colors 

 the particles a bright blue. The nuclei are comparatively small and less 

 numerous than in the upper part of the layer, or in the endoderm. In 

 addition to the granular, matrix-like cytoplasm, cells with coarse granules 

 occur here and there which stain differently from the other granules and 

 appear to be altogether distinct structural elements. In sections stained 

 with carmine and blue-de-Lyon they stand out a conspicuous red against the 

 ordinary blue granules. They rarel}^ if ever extend wholly across the layer 

 and seem comparable with the other coarsely granular cells found elsewhere 

 throughout the ectoderm and in the mesenterial filaments. 



Scattered throughout the calicoblast ectoderm are also a few small oval 

 nematocysts with a close spiral thread. Sometimes several may occur 

 together, but usually they are found singly. Different stages in their devel- 

 opment also can be observed. In the earlier stages they are large, with 

 homogeneous contents, and stain rather deeply, whereas when mature they 

 stain feebly, if at all. 



The presence of nematocysts within the calicoblast layer, where appar- 

 ently they can be of no service to the polyp, is somewhat remarkable. It is 

 to be borne in mind, however, that the calicoblast layer is but a modified part 

 of the ectoderm which, over the column wall and oral disc, always bears 



