6 NORMAL HISTOLOGY. 



Irritability is that property of living matter by virtue of which the eel! 

 exhibits changes in its form and intimate constitution in response to external 

 impressions. The latter may originate in mechanical, thermal, electrical or 

 chemical stimuli to which the protoplasm of even the lowest organisms 

 responds; or they may be produced in consequence of obscure and subtle 

 changes occurring within the protoplasm of neighboring cells, as illustrated 

 by the reaction of one neurone in response to the stimuli transmitted from 

 other nervous elements. 



CELL DIVISION. 



With the exception of the unusual cases in which division takes place 

 by the direct or amitotic method, the production of new generations of cells 

 of all kinds is accomplished by a complicated series of changes, collectively 



FiO. 5. Diagram of mitosis. A, resting stage, chromatin irregularly distributed in nuclear reticu- 

 lum ; a, centrosphere containing double centrosome ; n, nucleolus. E, chromatin arranged as close 

 spirem ; c, c, centrosomes surrounded by achromatic radial striations. C, stage of loose ?pirem, achro- 

 matic figure forming atnphiaster (amp). D, chromatin broken into chromosomes; nucleolus has dis- 

 appeare^, nuclear membrane fading; amphiaster consists of two asters (a, a) surrounding the separating 

 centrosomes, connected by the spindle (s). E, longitudinal cleavage of the chromosomes which are 

 arranged around the polar field (p) occupied by the spindle, f, migration of chromatic segments 

 towards new nuclei, as established by centrosomes (c, c); e p, equatorial plate formed by intermingling 

 segments. G, separating groups of daughter chromosomes (d, d) united by connecting threads (ct). H, 

 daughter chromosomes (d, d) becoming arranged around daughter centrosomes which have already 

 divided ; C, C, beginning cleavage of cytoplasm across plane of equatorial spindle. /. completed daughter 

 nuclei (D, D)\ cytoplasm almost divided into two new cells. (Modified from Wilson.) 



known as mitosis or karyokinesis, especially affecting the nucleus. In 

 addition to presiding over the nutritive changes within the cell, the nucleus 

 is particularly concerned in the process of reproduction; further, of the 

 various morphological constituents of the nucleus, the chromatin displays 

 the most active change, since this substance is the vehicle by which the char- 

 acteristics of the parent cell are transmitted to the new elements. So essen- 



