THE CELL 33 



in the surrounding protoplasm. The chromatin 

 becomes contracted and is seen as V-shaped loops 

 (chromosome), with thin closed ends pointed toward 

 the common centre, the polar field. The mother 

 stars are formed, which rapidly give origin to daughter 

 stars, in which the chromatin can be seen as two 

 separate nuclei grouped in a single mass of protoplasm; 

 at this stage the protoplasm becomes constricted 

 and two separate cells are seen lying in their own 

 protoplasm. (See Figs. 2 to 8, pages 36 and 37.) 



Cells of the animal and human body, or in fact all 

 living protoplasms, possess the properties of irritability, 

 conductivity, and motility. 



Irritability or the power of responding to some exter- 

 nal excitant. This can be mechanic, chemic, or electric ; 

 thus if the protoplasm acted upon be muscle, it will 

 contract; if a gland, such as the parotid, saliva will 

 be secreted; if a nerve, a sensation, as when we apply 

 heat, cold, etc., to the skin; or other nerve activity, 

 as seen in the contraction of the pupil when one looks 

 suddenly at a bright light. It must be remembered 

 that the degree of the response in the foregoing depends 

 upon the protoplasm acted upon and the nature and 

 strength of the irritating principle. 



Conductivity is developed best in muscle and nerves, 

 as seen when molecular disturbances occur at the ex- 

 tremity of the peripheral nerves, and are conducted 

 to the brain, and the same phenomena arising in the 

 brain are transmitted to the peripheral nerves. 



Motility is the power possessed by cells of apparent 

 active movement in response to natural causes, which 

 scientists have not yet determined. This motility is 

 best seen by microscopic technique and observed in the 

 ameboid movements of the white cells of the blood, 

 the waving of cilia, the activities of the spermatozoons 

 and ova during impregnation, or the commencement 

 of pregnancy, etc. 



