THE STRUOTURB OF XENIA HIOKSONE. 261 



SO that the part of the colony attached to the rock is hard and 

 quite different to the touch from any other part of the colony. 

 This hard base would afford a firm attachment and a rigid 

 support to the branches which arise from it. 



The spicules are formed within cells which at first lie in the 

 deeper parts of the ectoderm or have migrated into the outer 

 parts of the mesogloea (fig. 16). The arrangement of the 

 spicules is not very regular, as they appear to present to the 

 free surface their edge or their flat face indifferently. In 

 nearly all preparations the nucleus and remains of the proto- 

 plasm of the spicule-forming cell can be seen. The spicules, 

 which have a horny consistency, have an organic basis im- 

 pregnated with only a very small amount of calcareous matter. 

 They stain deeply with hsematoxylin, especially with hsemalum. 

 They do not dissolve when treated with acids, but shrink very 

 slightly, probably owing to the solution and extraction of the 

 small amount of calcareous matter which they contain. They 

 do not offer any difficulties in section-cutting, as the razor cuts 

 through them with little resistance, and sections 2 fx to 4 /x in 

 thickness may be readily obtained, although the specimen has 

 not been previously decalcified. On this account Xeuia offers 

 exceptional facilities for the study of the development of 

 spicules. As mentioned above, each spicule is formed within a 

 cell lying in the deeper part of the ectoderm. When the 

 young spicule first makes its appearance in the cell its substance 

 is scarcely distinguishable from the protoplasm of the cell; in 

 fact, it is not until the spicule has attained a diameter of 3 ^u to 

 4 IX that it is possible to clearly differentiate it (PI. 25, fig. 15). 

 The young spicule is then a small disc which stains with 

 hsematoxylin like the protoplasm of the cell, but its homo- 

 geneous structure enables the observer to distinguish it from 

 the finely granular cell-protoplasm which surrounds it. From 

 this stage the spicule grows regularly in length and thickness, 

 and the protoplasm of the cell covering it becomes gradually 

 thinner until, in a fully formed spicule, this protoplasmic 

 sheath forms an exceedingly thin investment, in which at one 

 part may be seen the small, somewhat flattened nucleus em- 



VOL. 42, PAKT 3. — NEW SERIES. S 



