Structure and Development of a Horny Sponge. 69 



{\<) The general mass of mesodermal tissue in which the 

 canals, flagellated chambers and otiier structures are 

 imbedded. — 



The flagellated chambers are pretty closely packed together 

 in the choanosome, and togetlier with the various branches of 

 the canal system, make up the greater part of its bulk. 

 Hence the amount of fundamental or ground tissue in which 

 they are imbedded is not very great. What there is is packed 

 full of minute, highly refringent graiudes, fairly evenly 

 distributed through it. Imbedded in this granular matrix 

 may be observed, here and there, small nucleated cells of 

 irregidarly rounded outline, doubtless the amoeboid cells of 

 authors. This ground tis.sue appears to agree thoroughly 

 Avith that which Schulze has described as existing in 

 Euspongia. 



(.")) The spongoblasts and other mesodermal cells 

 surroundinor the fibres. — 



In most parts the skeleton flbres are surrounded by a 

 sheath or" ordinary stellate or slightly fibrous connective 

 tissue. In some places, however, the stellate mesoderm 

 cells are specially modified as spongoblasts or glandular cells 

 whose function it is to secrete the spongin of which the 

 horny fibre is composed. The spongoblasts form a layer one 

 cell thick around the fibre. Each one is a somewhat club- 

 .shaped, slender, elongated, granular mesodei'mal cell, about 

 OO+S mm. long ; one end is drawn out into a long, gradually 

 tapering neck, and the other, broader end is usually rounded 

 ofi' but sometimes stellate, and contains a spherical nucleus. 

 The whole cell is frequently more or less bent or contorted. 

 Its long axis, however, always lies approximately at right 

 angles to the surface of the fibre against which its narrow 

 end abuts. There is commonly, if not alwaA's, a layer of 

 stellate mesoderm outside the layer of spongoblasts, and it 

 is easy to see that the spongoblasts themselves are simply 

 slight modifications of the ordinary stellate type of cell. 



II. — Development. 



My observations on the embryology of Stelospongus are as 

 yet very imperfect, for all the embryos which I have as yet 

 found are in pretty much the same stage of development. Of 

 this particular stage there is, however, an abundant supply, 

 and it presents some very remarkable features. 



