154 Prof. W. J. Sollas on the 



lies between two pores the ectodermal layers come nearly into 

 contact, though a few fine fibrils still seem to separate them ; 

 they clearly show, however, imbedded in their midst, and not 

 lying below them, the characteristic round nuclei with hamate 

 spicules surrounding them. These thin and narrow trabecule 

 seldom contain oval cells ; they are not wide enough ; but 

 these, along with granular cells, occur in the larger nodal 

 areas where three or more trabeculse meet (PL VII. figs. 9 

 &10). 



The edges of the pores are sometimes bordered by minute 

 granular fusiform cells with a minute spherical nucleolus in 

 an oval nucleus. 



Between the upper and lower ectodermal layers of the skin 

 is a layer of mesoderm of somewhat variable character, but 

 mainly consisting of a clear colourless jelly-like matrix, in 

 which are imbedded various cellular elements (PI. VII. fig, 2). 

 The most widely diffused, perhaps, are little circular or oval 

 rings -joVu i^ch in diameter, highly refractive, and of a pale 

 bluish tinge, enclosing round nuclei Tirorro inch in diameter, of 

 similar optical characters ; these, scattered irregularly through 

 the clear ground-mass, give it a curious appearance like 

 spotted muslin. Immediately beneath the annular cells 

 of the ectoderm succeed a number of separate, irregularly 

 rounded, granular greyish-coloured cells with round nuclei ; 

 they might very well be an early form of the annular cells. 

 Sometimes they form a layer two or three cells deep, some- 

 times thin out altogether. The remaining cellular constitu- 

 ents are fusiform granular cells, variously distributed ; lying 

 parallel side by side, they form the fibrous strands, which run 

 just below the epidermis to map out the pore-areas from each 

 other ; sometimes they form a thin layer beneath the surface, 

 in which they wander in all directions, and occasionally 

 extend singly at right angles to the surface from the upper to 

 the lower ectodermic layer. Apjiroaching the spicular 

 columns the dermal mesodermic layer thickens out, so that 

 the upper and lower ectodermic layers become gradually more 

 and more separated from each other — the upper rising tent- 

 like about the outer ends of the spicular columns, the lower 

 descending in a similar but inverted tent-like curve down the 

 continuations of the columns beneath the skin, and so round- 

 ing off the upper corners of the intermarginal cavities. In 

 correspondence with this thickening-out of the dermal meso- 

 derm, its fusiform fibres diverge fan-like as they enter the 

 spicular column, the more superficial ones running parallel 

 to their respective surfaces. The fusiform fibres in the 

 vicinity of the spicules run parallel to them, though near 



