CELL-FORMATION.

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case in question; and on the other, connects itself,through analogy, with other facts, as in the cell-formationalready mentioned, in Adilya, Zygnema, and other Algae ,as well as with the parietal cell-formation, where themembrane is likewise produced on the surface of thecontents.

The third type of free cell-formation still remains, inthe origin of a number of germ-cells of Algae, Fungi, andLichens . Here observation at first discloses to us ex-tremely small globular masses of mucilage. They becomelarger, granular, and at last an inclosing membrane maybe made out. Actual experience is again insufficient tofurnish a certain determination of the question when andhow the membrane originates. It seems to me safestto take the simple explanation of the facts which is firstsuggested by what we see, especially since they can inthis manner be most readily brought into agreement withthe other facts. I therefore assume here, that the mem-brane is formed round a collection of the mucilaginouscontents. Whether this happens earlier or later, seemsto be all one; but it is probable that the membraneactually exists some time before it is distinctly visible.The observations on free cell-formation therefore require,they partly allow, the assumption, that the membrane isproduced on the surface of the contents. This hypothesismust, since no facts and no analogy are opposed to it, holdas a universal law. The contents, therefore, are primary,in the cell, the membrane is only secondary.

In the production of the endosperm-cells a nucleusexists. The nucleus originates first. Subsequently, alayer of mucilage accumulates on its surface. Probablyit is the attractive force of the nucleus which draws to ita portion of the contents of the parent-cell. That thenucleus does possess such a power is known from manyfacts. In almost all cells in which a nucleus exists, aportion of the contents of the cell become collected on itssurface. Consequently, we may define cell-formation inthe embryo-sac in the following way :— A nucleus origi-

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