CELL-FORMATION. 113 



case in question; and on the other, connects itself, 

 through analogy, with other facts, as in the cell-formation 

 already mentioned, in Acldya, Zygnema, and other Algge, 

 as well as with the parietal cell-formation, where the 

 membrane is likewise produced on the surface of the 

 contents. 



The third type of free cell-formation still remains, in 

 the origin of a number of germ-cells of Algae, Fungi, and 

 Lichens. Here observation at first discloses to us ex- 

 tremely small globular masses of mucilage. They become 

 larger, granular, and at last an inclosing membrane may 

 be made out. Actual experience is again insufficient to 

 furnish a certain determination of the question when and 

 how the membrane originates. It seems to me safest 

 to take the simple explanation of the facts which is first 

 suggested by what we see, especially since they can in 

 this manner be most readily brought into agreement with 

 the other facts. I therefore assume here, that the mem- 

 brane is formed round a collection of the mucilaginous 

 contents. Whether this happens earlier or later, seems 

 to be all one ; but it is probable that the membrane 

 actually exists some time before it is distinctly visible. 

 The observations on free cell-formation therefore require, 

 they partly allow, the assumption, that the membrane is 

 produced on the surface of the contents. This hypothesis 

 must, since no facts and no analogy are opposed to it, hold 

 as a universal law. The contents, therefore, are primary, 

 in the cell, the membrane is only secondary. 



In the production of the endosperm-cells a nucleus 

 exists. The nucleus originates first. Subsequently, a 

 layer of mucilage accumulates on its surface. Probably 

 it is the attractive force of the nucleus which draws to it 

 a portion of the contents of the parent-cell. That the 

 nucleus does possess such a power is known from many 

 facts. In almost all cells in which a nucleus exists, a 

 portion of the contents of the cell become collected on its 

 surface. Consequently, we may define cell-formation in 

 the embryo-sac in the following way : A nucleus origi- 



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