152 Prof. W. J. Sollas o)i the 



in the nucleated membrane, each spicule surrounding a nucleus, 

 which does not differ in any a])parent respect from those of the 

 plate-like polygonal cells. There can be no question that 

 these minute spicules and their nuclei are associated parts ot 

 the same cell ; but how comes this cell into the ectoderm if 

 all spicular structures are a product of the mesoderm ? Can 

 a mesoderm-cell have wandered into the ectoderm "? and if one 

 kind of mesoderm-cell why not others? and then what be- 

 comes of the sharp distinction between these two layers ? The 

 simplest explanation would, of course, be that of mistaken 

 observation ; but, after repeated examination of my prepara- 

 tions, I can see no reason for admitting this. But this is not 

 all ; for in many places a thin annuLar wall appears about the 

 small round nuclei (PI. VII. fig. 9), whence results aflat oval 

 cell about -jTxnjo i^^ch in diameter, devoid of contents except for 

 a little clear colourless protoplasm, whicli may form a little 

 slightly granular heap about the nucleus and extend from it 

 to the outer wall ; the appearance of the lining membrane is 

 now that of a thin structureless film imbedding flat oval cells, 

 at intervals varying from that of their own diameter up to 

 close contact. No other structure can be seen more superfi- 

 cial than this, either when viewed face on or in transverse 

 section, and consequently, one can scarcely help regarding 

 it as the ectoderm ; it is at least conceivable, though not 

 probable, that it results from a change in the form of the 

 plate-like cells, consequent on immersion in spirits ; if these, 

 on shrinking, assume an oval form and become connected by 

 an exudation from the mesodei-m which subsequently har- 

 dens, an appearance similar to that described might be pro- 

 duced. 



The skin (PI. VII. fig. 7), as the outer covering membrane 

 may be briefly called, exhibits another change of structure. 

 This consists in the appearance of an immeasurably thin struc- 

 tureless membrane, whicli is in many places thrown into nume- 

 rous minute wrinkles having no apparent arrangement, except 

 when traced to the margins of tliepores,to which they are radiate. 

 It rises tent-like about the projecting spicules individually. 

 Associated with it, but lying on its under surface, as can be 

 clearly perceived by examining it in optical section where it 

 rises into tent-like projections or is most Avrinkled, are the 

 circular pale bluish nuclei of the subdermal epithelium, often 

 so regularly disposed at nearly equal distances from each 

 other, that one feels almost bound to regard the nuclei and 

 membrane together as an ectoderm from which the polygonal 

 outlines of cells have disap])eared — until one finds round some 

 of these nuclei, and lying on the underside of the membrane, 



