50 ESSENTIALS OF ZOOLOGY 



is possible to dissect the nucleus of a cell. Cells that are deprived 

 of their nuclei are unable to carry on assimilation, although catabo- 

 lism goes on until the cytoplasm is depleted. 



Cell Division 



The cell is limited in its size, as is the complete organism. This 

 limit of size is fixed primarily by the physiological necessities 

 which are transmitted through the surface of the cell. There is a 

 definite relation between volume and surface in any mass of mate- 

 rial, and this may be expressed in a ratio. With variation of the 

 size of the mass, the volume varies according to the cube of the 

 diameter while the surface area varies according to the square of the 

 diameter. When the limit of growth is reached the cell divides, 

 and this restores the proportion of the surface area to volume that 

 will again permit growth. Remak, in 1855, was the first to describe 

 cell division. His idea was that the nucleolus split first, then the 

 entire nucleus, and finally the cytoplasm divided, placing each por- 

 tion with its share of the nucleus. This direct method of division 

 was called amitosis. Its actual occurrence is quite rare. The usual 

 method of cell division is far more complex and less direct. There 

 are several preliminary changes or phases which must occur before 

 the actual cleavage of the cell into two new ones. This is mitotic 

 cell division, more briefly mitosis, or indirect cell division. This 

 method of division was first described by Fleming in 1878, though 

 Schneider in 1873 described much of the complicated process. 



Although the process of mitosis is a continuous series of changes, 

 for convenience in study, these changes will be set out as six phases. 

 Following the resting cell condition come the first changes, and the 

 early prophase condition is seen. In this stage the centriole has 

 divided, and the two pieces have moved considerably apart. The 

 surrounding protoplasm has produced some rays radiating from each 

 centriole. These two bodies are now known as asters because of their 

 starlike appearance. The two asters taken together are called the 

 amphiaster. The nucleolus disappears and the chromatin becomes 

 organized into a long, tangled fiber, which is called the spireme 

 thread. 



In the middle prophase stage the centrioles have migrated still 

 farther from each other, and the spindle fibers between the centrioles 

 as well as the astral rays around them have become well established. 

 The spireme thread has thickened and divided into segments in the 



