26 



STUDIES IN ADVANCED PHYSIOLOGY. 



Fig. 4 . AMCEBA. (After Leidy.) 

 n, nucleus; cv, contractile vacuole 

 (excretory organ); N, food vacuoles; 

 ek, exoplasm ; en, endoplasm. 



Fig. 5. A TYPICAL CELL FROM THE INTES- 

 TINAL EPITHELIUM OF A WORM. (Carnoy.) 

 me, membrane of cell ; pc, protoplasm of the 

 cell ; mn, membrane of nucleus ; pn, achroma- 

 tin of the nucleus; bn, the chromatin filament 

 about to change into the separate chromo- 

 somes. 



the formation of a new cell, is believed to be the carrier of 

 those qualities and properties which we are wont to desig- 

 nate as those of heredity. In cells which have lost the 

 power of division the nucleus practically disappears, but it 

 is conspicuous in eggs which are about to usher in a period 

 of active cell division. By a chemical modification of the 

 protoplasm of the cell, the cell is differentiated into the re- 

 quired tissue in question. By changes of some kind in the 

 protoplasm of the cells are brought about all those phenom- 

 ena of physiology such as secretion, absorption, assimilation, 

 contraction, and some physiologists go on to add conscioiis- 

 ness and thozight, a step by no means yet fully warranted. 



These various activities of the cells will be discussed 

 in their appropriate chapters, and attention is called here 

 briefly only to the somewhat complicated, but interesting 

 process of the indirect division of the cell. 



THE DIVISION OF THE CELL. 



When the process of cell division, called karyokinesis, is 

 about to be ushered in, the centrosome lying close to the 



