

PHYSIOLOGY OF THE CELL. 29 



external excitation, whether mechanical, chemic, or electric, is a fundamental 

 property of all living protoplasm. The character and extent of the reaction 

 will vary, and will depend both on the nature of the protoplasm and the 

 character and strength of the stimulus. If the protoplasm be muscle, the 

 response will be a contraction; if it be gland, the response will be secretion; 

 if it be nerve, the response will be a sensation or some other form of nerve 

 activity. 



Conductivity, or the power of transmitting molecular disturbances arising 

 at one point to all portions of the irritable material, is also a characteristic 

 feature of all protoplasm. This power, however, is best developed in that 

 form of protoplasm found in nerves, which serves to transmit, with extreme 

 rapidity, molecular disturbances arising at the periphery to the brain, as 

 well as in the reverse direction. Muscle protoplasm also possesses the 

 same power in a high degree. 



Motility, or the power of executing apparently spontaneous movements, 

 is exhibited by many forms of cell protoplasm. In addition to the molecular 

 movements which take place in certain cells, other forms of movement are 

 exhibited, more or less constantly, by many cells in the animal body e. g., 

 the waving of cilia, the ameboid movements and migrations of white blood 

 corpuscles, the activities of spermatozooids, the projections of pseudopodia, 

 etc. These movements, arising without any recognizable cause, are fre- 

 quently spoken of as spontaneous. Strictly speaking, however, all proto- 

 plasmic movement is the resultant of natural causes, the true nature of which 

 is beyond the reach of present methods of investigation. 



Reproduction. Cells reproduce themselves in the higher animals in 



two ways by direct division and by indirect division, or karyokinesis. 



In the former the nucleus becomes constricted, and divides without any special 



grouping of the nuclear elements. It is probable that this occurs only in 



disintegrating cells, and never in a physiologic multiplication. In division 



by karyokinesis (Fig. 3) there is a progressive rearranging and definite 



grouping of the nucleus, the result of which changes is the division of the 



centrosome, the chromatin, and the rest of the nucleus into two equal portions, 



which form the nuclei. Following the division of the nuclei, the protoplasm 



divides. The process may be divided into three phases: 



i. Prophase. The centrosome, at first small and lying within the nucleus, 



increases in size and moves into the protoplasm, where it lies near the 



nucleus, surrounded by a clear zone, from which delicate threads radiate 



through an area known as the attraction sphere. The nucleus enlarges and 



becomes richer in chromatin. The lateral twigs of the chromatin cords 



are drawn in, while the main cords become much contorted. These cords 



