UTRICULAR STRUCTURES.

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so far as to enable us to distinguish the organic molecules,there will always be still entire stages, in the developmentof membranes, in which it will be impossible to distinguishthem from the homogeneous contents lying close to them,and refracting light in a similar manner, and in whichtheir presence or absence must be decided on upon othergrounds.

The matter stands in a similar position in regard tocells. The relation of contents and membrane is evidentin fully-formed cells, and it may be carried back by aconclusion from analogy to the young cells. The youngcells in the embryo-sac, if they possess homogeneous andnot granular contents, often do not allow of our perceiv-ing the membrane for a long time. Free germ-cells(spores) of Algae, Fungi, and Lichens , mostly attain aconsiderable size (at least as considerable as that of thenucleus mentioned by Schleiden ), without the possibilityof seeing the least sign of membrane, and yet they areyoung cells, to which no one will deny a membrane.*Moreover, cells occur also, about the size of young nuclei,on which no membrane is visible in their lifetime. Amongthese arc some species of Protococcus, of Palmella, and ofother Falmelleae.

If, then, it is impossible to distinguish the membranefrom the contents in young free cells, or in small freecells generally, on the other hand the membrane is usuallyvisible in parietal cells at the moment when the celloriginates, and in those very places where the appositionof the secondary cells forms a septum. The same occursin nuclei which originate by the division of a parent-nucleus. Here, too, as in the division of cells, a septumis visible, formed by the two meeting membranes.f

The comparison of those free cells is especially neces-sary here, in which membrane -and contents cannot beclearly distinguished even in the fully-developed con-

* See pages 95-6 of this volume.

t Niigeli on Cells, Part I, Ray Trausl. 1S15, pp. 231, 216.