DESCRIPTION OF EPHYDATIA BLEMBINGTA. 91 



coat as to present a perfectly smooth and membrane-like 

 appearance. 



(2) The Origin and Subsequent Changes of the 

 Cells which produce the Ground Substance of the 

 Protective Coat of the Gemmule. — These cells, after 

 having wandered from the general sponge tissues, appear in 

 the neighbourhood of the gemmule as a loosely arranged layer 

 situated outside the future yolk cells. Fig. 11a (PI. 2) 

 shows how they travel towards the developing gemmule and 

 how they become concentrated to form a layer. 



Their general protoplasm is clear, but they contain a number 

 of granules or yolk bodies which resemble those of the yolk 

 cells. In addition, they often contain a much bigger spherical 

 body which seems to be of the same nature as what I have 

 described in my account of the structure of the larva of 

 Spongilla lacustrisas nutritive vacuoles. The cells which 

 develop to yolk cells seem never to contain either of the 

 above bodies at their first appearance. At first they are 

 spherical in shape, but soon become columnar, though never 

 club-shaped. However, their outer end may be round and 

 not flat during certain stages (PI. 3, fig. 13, and PI. 4, fig. 15a). 

 They assume the columnar form, at first, only on one side of 

 the reproductive mass of cells, the columnar layer so formed 

 gradually increasing in extent until it completely surrounds 

 the yolk cells. The point at which the columnar layer is 

 finally completed marks the position of the future pore of the 

 gemmule. 



Subsequent to the assumption of the columnar form, these 

 cells begin to secrete the inner chitinous layer, which in its 

 formation follows the same course as the columnar layer did, 

 which is a proof that the layer in question is secreted by the 

 columnar cells and not by yolk cells (PI. 3, fig. 13; PI. 4, figs. 

 14 and 15). 



Soon after the amphidiscs have taken up their position 

 among the columnar cells — a phenomenon which takes place 

 soon after the formation of the columnar layer — the latter 

 grow out and before long appear outside the outer ends of 



