104 RECORD OF CURRENT RESEARCHES RELATING TO 



The fihre-layer is a layer separated from the epithelium by 

 a layer of small and one of large stellate spicules. It consists in 

 the youngest embryos (gemmte) of oval, granular, nucleated cells 

 lengthened fusiformly, of about the same size as the epithelial cells. 

 In embryos of twice the size they have become slightly larger and 

 more elongated. In the adult they may (1) assume a true Jihre-shape 

 by elongation, containing a nucleus and granular protoplasm, or 

 (2) remain spindle-shaped, the protoj)lasm remaining clear, but being 

 longitudinally fibrillated. 



The medullary tissue occupies the centre of the sponge, lying 

 immediately beneath the preceding layer, and consists of an aggrega- 

 tion of cells which may be isolated and are distinguishable in the 

 tissue itself; their nuclei often contain nucleoli. In the younger 

 embryos they are roundish, oval, granular, with a large nucleus ; 

 they are almost half as large again in the older embryos ; in the 

 adult they are found again to possess the former dimensions 

 ('0069 mm. long) and have an angular, elongated form, their nuclei 

 often becoming invisible or being lost (it does not appear which is 

 really the case). 



The water-vascular system is absent in gemmae of 1 mm. diameter. 

 In those of 2 mm., however, the diflferent tissues, as already enumerated, 

 are developed ; and within these, between the large-stellate-spicule 

 layer and that of the small spicules, which lies directly beneath the 

 epidermis — run the main water-canals. These send off branches with 

 regularity to the surface, which is uniformly perforated by the pores, 

 and to the central medulla. The ciliated endothelium cells which 

 line its course rest on small stellate spicules, which in turn are 

 surrounded by fibre-cells. To these arrangements of contractile 

 fibres and of the vessels must be due the circulation of water through 

 the system, and its sudden expulsion, accompanied by contraction of 

 the body, when the sponge is removed from the sea. 



The linear spicules range from 'OITG to '0315 mm. in breadth in 

 different gemmfe ; three varieties between these limits exist in the 

 adult. They are enveloped in a bi-laminate sheath ; the inner layer 

 being the proper sheath-layer, the other being formed by the sur- 

 rounding sponge-cells ; the former is distinguished, as discovered by 

 Keller, by being more deeply stained by reagents than are the sur- 

 rounding cells ; its cells are also much smaller than these. In young 

 gemmas the sheath covers the point as well as the body of the spicule. 



The small stellates apjiear in the outer part of the fibre-layer, first 

 in one, ultimately in many rows; they are each developed from a 

 single cell, the nucleus of which is converted into the spicule ; in 

 their youngest stages they dissolve in boiling hydrochloric acid. 



The large stellates appear first in gemmse of 2 mm. diameter, 

 and form a single row beneath the last-named forms; they are 

 developed in "giant-cells" of -0315 mm. diameter, from the cell- 

 nuclei. 



The gemmae appear first in the small-stellate layer, as aggrega- 

 tions of four specialized cells each, lying in a capsule composed of 

 long fibre-cells. Of the four embryonic cells, three originate the 



