l6 ANIMAL FUNCTIONS OF CELLS. [SECT. 1 5. 



larger vessels, perhaps belong to this category, in so far as they 

 appear to arise from the secretion of fluid in the interior of primi- 

 tively compact cell-masses. 



The application of the doctrine of the double cell-membrane in vegetable 

 cells, the primordial utricle {Mold) and cellulose membrane, to animal-cells, 

 took place in 1852, contemporaneously and independently, by Remak (Mull. 

 Arch. 1852, p. 63, scq.) and myself {Handb. d. Oeweb. 1852, pp. 14, 29). Since 

 this time, my observations en the cuticular structures {Transact of the Phys. 

 Med. Soc. of Wurzburg, viii.) have shown that secondary depositions from the 

 cells, analogous to the cellulose membrane of vegetable-cells, are to be found 

 in a great many places, and are often characterised by a very particular 

 structure, especially by the existence of a large number of extremely fine 

 pores, pervading them in the direction of their thickness. 



§ 15. Animal Functions of Cells. — To the vital phenomena of 

 cells, also belong certain movements which appear in cells, of which 

 it is extremely difficult to say whether they concern only the con- 

 tents, or the cell-membranes also. They are most simple in those 

 lower animals which have the signification of simple cells. Here 

 there exists an entire group, Rhizopoda {Amoeba, Arcella, Difflu- 

 gia, etc.), the substance of whose bodies, without presenting a 

 differentiation between envelope and contents, is capable of assum- 

 ing the most diverse shapes. A similar amorphous "contractile 

 substance" (Ecker), which also may be designated Sarcode (Du- 

 jardin), also occurs in the Protozoa provided with a special outer 

 membrane (cell-membrane), and here occasions the changes of the 

 contractile spaces, the movements of the pedicle of the Vorticellce, 

 and probably also the currents of the fluids, .such as are found in 

 Loxodes bursaria. In these animals, the outer envelope also 

 appeal's to be contractile, either in its totality, or in its external 

 processes, the cilia ; yet it is also conceivable that all the 

 movements are only dependent upon the contents, and that the 

 envelope simply follows them as an elastic body. On this view, 

 a cilium must be conceived as being a pedicle of a vorticella in 

 miniature ; in which latter, as Czermdk shewed, the inner filament 

 connected with the substance of the body is contractile, while the 

 envelope is elastic. 



In the higher Animals, contractile phenomena of this kind are 

 found, firstly, in individual cells; and then in parts of tissues 

 which owe their origin to a metamorphosis of single cells. To the 

 former belong, 1. Cilia, external processes of cells, outgrowths 

 which probably are not only to be regarded as prolongations of 

 the cell-membranes, but also of the contents ; so that it cannot be 

 said, whether their power of contraction is to be referred to the 



