UTRICULAR STRUCTURES.
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utricles producing starch and chlorophyll, in Caulerpa .*As a criterion of their utricular nature, I mentioned thatthey do not originate by deposition on the outside, thatthey inclose, in a complete membrane, contents distinctfrom this membrane, fluid, semifluid, or, sometimes, par-tially granular, that they grow by expansion of theirmembrane, and transform their contents, and that theypropagate in the same way as cells.
In their essential peculiarities, therefore, these utriclespresent the character of cells; and we shall consequently,in the decision whether anything is an utricle or not, takeespecially into account the phenomena which are analogousto the phenomena of cell-life. On the other hand, we shallnot determine whether anything is to be considered as anutricle by the circumstance of its being hollow or solid.Now, as cells occur which appear solid, either becausetheir very delicate membrane is quite filled with contents,or because they are completely lignified, so also do suchutricles present themselves. Moreover, hollow structuresoccur in the cell-contents which are not utricles.
I must enter more minutely into the latter point,because it has very often led to error. Homogeneousmucilaginous contents frequently exhibit transparent, co-lourless cavities. These are globular when they are iso-lated, or at least are not very near together ; they becomeparenchymatous when they are closely crowded. Theyvary much in size ; sometimes a great number find placein one cell, sometimes one, two, or three, occupy almostthe entire cavity.
The mucilage in which these cavities appear is eitherof equal density throughout, or it is denser at the peri-phery of the cavities, and forms as it were a membranearound them. In such cases the cavities appear likeutricles. This utricular structure is the more deceptivethe more the density of the mucilage immediately on thesurface of the cavity differs from that of the rest of theinterior of the cell; it is still more deceptive when these
* Loc. cit. p. 149.