THEORY OF FOOD- ABSORPTION. 57 



interior. Such parts of foreign bodies as are not serviceable for nutrition are sub- 

 sequently eliminated or are left behind by the protoplast as it creeps onward. But 

 this method of food-absorption is limited to amoeboid forms belonging to the 

 boundary-land of animal and vegetable life. The movements of other naked proto- 

 plasts, such as those which are carried about in the water by vibratile cilia, have 

 nothing to do with the search for food or with its absorption, but are connected 

 rather with the processes of distribution and propagation. 



THEORY OF FOOD-ABSORPTION. 



In the case of protoplasts inclosed in cell-membranes the food necessary for 

 nourishment must always pass through the cell-membrane and peripheral proto- 

 plasmic layer (ectoplasm) into the interior of the protoplasmic bodies. And so, 

 conversely, such of the substances absorbed as are of no use in the construction 

 of the organism or for any other purpose, must be separated and passed out 

 through these envelopes. The cell-membranes of those protoplasts which are 

 employed in absorbing food must accordingly have a special structure: the 

 ultimate particles must be so arranged as to allow of the passage of nutritious 

 material inwards, and of rejected matter outwards, without prejudice to their own 

 stability. The passages in cell-walls used for this purpose are very minute, much 

 smaller at all events than the pore-canals described above as being occupied by 

 fine protoplasmic filaments; the dimensions are in fact so trifling as to be invisible 

 even with the best microscopes. Still we are forced to conclude that they exist 

 by a posteriori reasoning from a series of phenomena, and to assume that the cell- 

 membrane, like almost every other kind of body, consists not of continuous matter, 

 but of minute particles, which are termed atoms, and are separated from one 

 another by infinitesimally small spaces. Various processes and appearances have 

 also led physicists and chemists to the conclusion that these atoms are not aggre- 

 gated in disorder, but are always combined together in groups of two or more, 

 even in the case where all the atoms in a body are of the same kind, i.e. are the 

 same element. If a body contains different elements they are not mixed together 

 indiscriminately, but are grouped in conformity to a definite law: every group 

 includes atoms of all the different elements concerned, arranged in a certain in- 

 variable manner, not only as regards number, but also as regards relative position. 

 Groups of atoms of this kind are called " molecules," and the spaces between them 

 are supposed to be larger than those between single atoms. Further, it is not 

 improbable that the molecules themselves form groups, each group consisting of 

 molecules conglomerated in a definite manner, and that the passages separating 

 these molecular groups are larger again than those separating the single molecules 

 within each group. These groups of molecules have been called "micellae" or 

 Tagmata, and they also are supposed to be aggregated together in definite order. 



According to this theory the cell-membrane is analogous to a sieve, the pores 

 of which are grouped in a definite manner, the broadest perforations being between 



