74 Prof. H. James-Clark on the American Spongilla 



each other at various angles. To complete this general sketch, 

 we will state more definitely the relation of the constituents of 

 the monadigerous mass. There are essentially but two ele- 

 ments here, — namely, the inner division (c) of the investing 

 membrane, and the groups of monads {h) which are imbedded 

 in it below its surface. In a fully expanded individual these 

 groups seldom lie so closely as to touch each other. They 

 vary considerably in size and are usually globular or spheroidal 

 and form a single stratum^ with rather narrow interspaces (c^) 

 between them. 



It seems proper here, at least for the sake of precision, that 

 the cytohlastematous basis, in which the monad groups are im- 

 bedded, should be considered apart from the epithelium-like 

 inner (c) investing membrane which overlies it, although the 

 two are essentially one, the epithelioid membrane, by prolong- 

 ing itself between (at c^) and beneath the groups, forming for 

 them a continuous foundation. In this light, then, we shall 

 speak of the monadigerous mass as consisting of three elements, 

 • — namely, the inner investing membrane proper, the group of 

 monads, and the cytohlastematous basis. This basis seems to 

 constitute a large part of the bulk of the body, since it occupies 

 all of the interior space beneath the monad groups. In spe- 

 cimens which grow over flat surfaces in depressed patches, or 

 around stems of plants, it forms a relatively thin layer ; but 

 where the body stands out as an irregularly rounded mass, 

 sometimes an inch in diameter, the cytohlastematous basis 

 fills up the interior, in enormous proportion to the bulk of the 

 monad layer. 



Organography. 



The Investinc/ Membrane. — The investing membrane (fig. l,a 

 a}^cd) consists essentially of two histiological elements — namely, 

 a very diffuse cytoblastema {a^), and irregularly disposed cells 

 {b, b^, b^) scattered through it. The intercellular cytoblastema 

 forms a very thin layer (a') between the cells (b) ; but where 

 the latter are imbedded in it, its outer and inner faces are as 

 wide apart as the considerable depth of the cells demands ; and 

 thus it happens that the membrane (both the outer and the 

 inner divisions) presents in profile (a\ c, d) such an irregular 

 thickness. The cytoblastema (a^) is colourless, hyaline, and ap- 

 parently homogeneous under a low power; but when magnified 

 to about four hundred diameters, it displays a very finely 

 granular aspect. It occupies wide intervals between the cells, 

 certainly more than one half, and fully three fifths of the whole 

 area of the membrane. Its apparent extent, in a general view, 

 is even more than that, owing to the extreme transparency of 



