4 THE CELL AND SIMPLE ANIMAL TISSUiCS. 



Masses of a slimy substance — probably protoplasmic — in 

 nature, named Bathyiius by Huxley, are said to be found on 

 the floor of the Atlantic Sir Wyville Thomson, who at one 

 time believed in the protozoon nature of this Bathyhius, later 

 asserted that it is in reality a gelatinous precipitate of calcium 

 sulphate thrown down by the action of alcohol upon sea-water, 

 — the dredgings in which the slime was detected having been 

 preserved in spirit. Protoplasm is certainly foimd unprotected 

 by any surrounding membrane in some of the simplest Protozoa. 

 This all-important living substance is endowed with the powers 

 of contractility and movement, and is subject to such external 

 influences as light, heat, and electricity. Movement takes place 

 by the protrusion of any part of its surface, the protruded parts 

 being known as " pseudopodia," the rest of the protoplasm 

 flowing in a wave-like manner after these processes. 



Inside the protoplasm of a cell is a body called the nucleus, 

 which may be either a solid mass of protoplasm or a more fluid 

 mass enclosed in a membrane, and containing one or more solid 

 . particles called nucleoli. The nucleus is composed of a more 

 liquid part, the "nuclear fluid," and a more solid part, the 

 " nuclear protoplasm." The nucleus varies in form : sometimes 

 it is round, at others oval, or again it may be elongated and 

 twisted. 



Protoplasm and nucleus are surrounded, as a rule, by a cell- 

 wall, a definite bounding membrane, which mm'e or less retains 

 the contractile protoplasm. The essential part of the cell is the 

 protoplasm, which has the power of independent movement, 

 of metabolism, and of reproduction. Most organisms that we 

 shall deal with, excepting the simple Protozoa, will be seen to 

 be made up of numbers of these cells, which become united in 

 various ways, and so form the animal tissues. We now know 

 that a cell always originates from a pre-existing cell. The 

 formation of one cell from another takes place chiefly by a 

 process known as haryoMnesis or " cell-division." 



When a cell has received its full share of nourishment — that 



