16 tHK ANIMAL CELL [CH. It. 



by a jet of steam allowed to play for a moment on the surface 

 of the cover glass. The next figure illustrates one fixed in this 

 way. 



The essential act in the protrusion of a pseudopodium is the 

 flowing of the hyaloplasm out of the spongioplasm ; the retraction 

 of the pseudopodium is a return of the hyaloplasm to the spongio- 

 plasm. The spongioplasm has an irregular arrangement with open- 

 ings in all directions, so that the contractility of undifferentiated cells 

 may exhibit itself towards any point of the compass. 



The relation of cells to various forms of stimulus has been recently very 

 extensively studied. Various forms of unicellular organisms have been used in 

 these experiments, and the stimuli employed have been chemical, thermal, light, 

 electric currents, and so forth. If the cell moves towards the source of attraction, 

 the term positive taxis is employed ; if it is repelled, negative taxis. The words, 

 chemo-taxis, thermo-taxis, photo-taxis, galvano-taxis, etc., indicate the kind of 

 stimulus investigated. 



Cell Division. 



A cell multiplies by dividing into two; each remains awhile 

 in the resting or, more correctly, non-dividing condition, but 

 later it grows and subdivides, and the process may be repeated 

 indefinitely. 



The supreme importance of the cell, the growth of the body from 

 cells, and the fact that cells are the living units of the organism, 

 were first established in the vegetable world by Schleiden, and 

 extended to the animal kingdom by Theodor Schwann. The ideas 

 of physiologists depending on this idea are grouped together as 

 cellular physiology, which under the guidance of Virchow was ex- 

 tended to pathology also : Virchow expressed the doctrine now so 

 familiar as to be almost a truism in the terse phrase omnis cellula e 

 cellula (every cell from a cell). 



The division of a cell is preceded by division of its nucleus. 

 Nuclear division may be either (1) simple or direct, which consists in 

 the simple exact division of the nucleus into two equal parts by con- 

 striction in the centre, which may have been preceded by division of 

 the nucleoli ; or (2) indirect, which consists in a series of changes 

 which goes on in the arrangement of the nuclear reticulum, resulting 

 in the exact division of the chromatic fibres into two parts, which 

 form the ohromoplasm of the daughter nuclei. 



The changes in the nucleus during indirect division constitute 

 karyokinesis (icapvov, a kernel), or mitosis (///TO?, a thread), and 

 direct division is called amitotic or akinetic (Kivyaris, movement). It 

 is now believed that the mitotic nuclear division is all but, though 

 not quite, universal. Somewhat different accounts of the stages of 

 the nuclear division have been given by different authorities, accord- 

 ing to the kind of cell in which the nuclear changes have been 



