Mr. H. J. Carter on the Ultimate Structure of Spongilla. 23 



because it resembles the vacuolar state of the protoplasm in the 

 early development of the cell of the Characese^ and probably of 

 vegetable structures generally. The margin of this border is 

 more or less irregular and digitated, from the polymorphic sub- 

 stance of which it is composed ; while further in may be seen 

 the ovi-bearing cells in the denser gelatinous matter, with their 

 ovules already somewhat diminished in size. The spicules have 

 also begun to appear. 



Second Period. — We may commence with the formation of 

 the spicules, which is so rapid, that they come into view almost 

 simultaneously with the issue of the substance of the seed-like 

 body. 



Spicules. — These appear to be formed in sponge-cells of a 

 peculiar kind, one of which is confined to the parenchyma, viz. 

 that which forms the large smooth spicule, and the other to the 

 seed-like bodies and the investing membrane, viz. that which 

 forms the small spiniferous spicule characteristic of the species. 

 Of the former I can state nothing, except that it appears to be 

 filled with ovules like the ovi-bearing cell, while the latter is 

 characterized by the absence of ovules, a uniform granular com- 

 position, and the presence of a nucleus (PI. I. fig. 7). 



At the earliest period that a spicule becomes visible it appears 

 under a hair-like form of immeasurable thinness, and enclosed 

 in a sponge-cell of a spindle-shape, which has assumed this 

 figure to accommodate it. The nucleus of the cell is now seen 

 in its centre, and the spicule, about ^^q^^ ^^ ^^ ^^^^ ^^ length, 

 lying across it (fig. 8 a'). After a few hours the spicule becomes 

 much larger and longer, and the sponge-cell still more extended 

 to retain it within its substance ; it also presents a glairy, ovate 

 globule in its centre, through which its shaft passes, not in the 

 line of the longitudinal axis, but on one side of it, so that the 

 globule looks as if it were appended to it (fig. 8 a, 6, c). When, 

 however, the spicule is arrested at this stage of its development 

 and denuded of the sponge-substance, that part which in the 

 sponge-cell appeared to be a glairy, refractive globule, is found 

 to be merely an inflation of the outer wall of the spicule, for the 

 shaft of the spicule, slightly diminished in size, may then plainly 

 be seen to pass through it in the manner before mentioned, and 

 to present the longitudinal canal in its inside. In this state 

 neither undiluted nitric acid nor a saturated solution of caustic 

 potash produces any change in it, so that it may fairly be as- 

 sumed to be of the same composition as the rest of the spicule. 



By degrees, as the spicule is enlarged, the inflation is also 

 proportionally increased in size, and disappears only when the 

 spicule is fully formed (fig. 8 d). The normal state of the in- 

 flation appears to be single and in the centre of the spicule, but 



