PHYSIOLOGY OF THE CELL. 



tozooids, the projections of pseudopodia, etc. These movements, 

 arising without any recognizable cause, are frequently spoken of as 

 spontaneous. Strictly speaking, however, all protoplasmic move- 

 ment 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 karyo- 

 kinesis. In the former the nucleus becomes constricted, and divides 

 without any special grouping of the nuclear elements. It is prob- 



Close Skein Loose Skein (viewed 

 (viewed from from above i. e., from 



the side). 

 Polar field. 



the pole). 



Mother Stars (viewed from the side). 



Mother Star (viewed Daughter Star, 

 from above). 



Beginning. Completed. 

 Division of the Protoplasm. 



FIG. 3. KARYOKINETIC FIGURES OBSERVED IN THE EPITHELIUM OF THE ORAL 

 CAVITY OF A SALAMANDER. 



The picture in the upper right-hand corner is from a section through a divid- 

 ing egg of Siredon pisciformis. Neither the centrosomes nor the first 

 stages of the development of the spindle can be seen by this magnification. 

 X 560. 



able 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 



