16 THE CELL 



We know nothing at all about how life first appeared on the earth, and we 

 are unable to attach any great importance to the hypotheses which have been 

 put forward concerning its origin. For a long time it was imagined that 

 many kinds of living creatures arose directly from dead matter by spontaneous 

 generation. But the more deeply studies in this direction were followed, the 

 more improbable this view became., and at last it was held only with reference 

 to the lowest organisms; until finally Pasteur (1861) by his ingenious re- 

 searches incontestably established the fact that spontaneous generation does 

 not take place at all. 



Within the cell the living substance is divided between the nucleus and 

 the surrounding protoplasm. The protoplasm may be manifoldly differenti- 

 ated into contractile fibers, cilia, etc. Besides, the cell contains in greater 

 or less quantity nonliving substances of the most different kinds, some of 

 which appear clearly as specialized contents e. g., the cell-sap of plant 

 cells and the fat of fat cells while some are intimately mixed with the 

 nucleus or protoplasm and are therefore not to be distinguished from living 

 substance. 



Some cells are surrounded by a specialized cell membrane, while others have 

 none ; hence it is not an essential constituent of a cell. Almost all plant cells have 

 even in their early stages a specialized membrane which increases in thickness 

 as the cell grows in size. According to present views it represents either a 

 transformation product of the outer layer of the protoplasm or a secretion 

 product of the cell. Only a few animal cells have an actual membrane. The 

 zona pellucida of the egg cell, the membrane of fat cells and probably the 

 sarcolemma of muscle fibers are about all that can be named. In other animal 

 cells however the outermost layer of the cytoplasm is often firmer and more 

 elastic than the inner parts, and is therefore able to protect the cell to a certain 

 extent in the same manner as a true membrane. 



Cells differ greatly in external form. The spherical form which we regard 

 as the type is by no means general : we find on the contrary a great variety 

 of forms, not only in the many-celled organisms, where the shape of the cell 

 is influenced by its position with reference to other cells, but also in free 

 living, isolated cells. Cells likewise vary in size, all the way from that which 

 is perceptible only with the highest magnification of the microscope to that 

 of the giant cells of certain algae, many meters in length. 



The nucleus occurs usually as a spherical or oval body in the middle of 

 the cell; but it may take many other forms. As a rule the size of the nu- 

 cleus bears a direct proportion to the size of the cell. The larger the cell is, 

 the larger is the nucleus. However, there are many exceptions to this rule 

 also. Most cells contain but a single nucleus, although not infrequently two 

 or more may be present. Indeed, in the giant cells of the bone-marrow, in 

 several of the lowest organisms, and in some other cells, as many as one 

 hundred nuclei have been observed. 



