UTRICULAR STRUCTURES. 163 



utricles producing starch and chlorophyll, in Caulerpa* 

 As a criterion of their utricular nature, I mentioned that 

 they do not originate by deposition on the outside, that 

 they inclose, in a complete membrane, contents distinct 

 from this membrane, fluid, semifluid, or, sometimes, par- 

 tially granular, that they grow by expansion of their 

 membrane, and transform their contents, and that they 

 propagate in the same way as cells. 



In their essential peculiarities, therefore, these utricles 

 present the character of cells ; and we shall consequently, 

 in the decision whether anything is an utricle or not, take 

 especially into account the phenomena which are analogous 

 to the phenomena of cell-life. On the other hand, we shall 

 not determine whether anything is to be considered as an 

 utricle by the circumstance of its being hollow or solid. 

 Now, as cells occur which appear solid, either because 

 their very delicate membrane is quite filled with contents, 

 or because they are completely lignified, so also do such 

 utricles present themselves. Moreover, hollow structures 

 occur 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. Homogeneous 

 mucilaginous contents frequently exhibit transparent, co- 

 lourless cavities. These are globular when they are iso- 

 lated, or at least are not very near together ; they become 

 parenchymatous when they are closely crowded. They 

 vary much in size ; sometimes a great number find place 

 in one cell, sometimes one, two, or three, occupy almost 

 the entire cavity. 



The mucilage in which these cavities appear is either 

 of equal density throughout, or it is denser at the peri- 

 phery of the cavities, and forms as it were a membrane 

 around them. In such cases the cavities appear like 

 utricles. This utricular structure is the more deceptive 

 the more the density of the mucilage immediately on the 

 surface of the cavity differs from that of the rest of the 

 interior of the cell ; it is still more deceptive when these 



* Loc. cit. p. 149. 



