CHAPTER I. 



THE CELL. 



The smallest unit of living tissue, both plant and animal, is the cell. 

 Every organism is a cell, a group of cells, or an aggregate of cells with 

 certain other structures produced directly by the activity of cells. Some 

 plants, and some animals, consist permanently of a single cell each. Every 

 plant and every animal commences its individual life as a single cell. 

 These unicellular organisms possess, in a limited way, the functioning 

 capacities of more complex organisms. The study of any form or func- 

 tion of living tissue may therefore advantageously begin in a study of the 

 cell. [Fig. 1.] 



2 



3 



•* 

 S 



6 



7 



Fig. I. Diagram of typical cell. (Bailey, Histology.) I. Cell membrane. 2. 

 Granules of metaplasm. 3. Net-knob, or karyosome. 4. Hyaloplasm. 5. Spongio- 

 plasm. 6. Linin net-work. 7. Nucleoplasm. 8. Attraction-sphere. 9. Centrosome. 

 10. Plastids. 11. Chromatin net-work. 12. Nuclear membrane. 13. Nucleolus. 14. 

 Vacuole. 



The cell has been denned as " a mass of protoplasm containing a nu- 

 cleus." * It is true, we find certain cells, the red blood corpuscles of 

 mammals [Fig. 3] which, during the period of their special functional 

 activity, have no nucleus. These cells, however, have finished their 

 growth, and die without issue ; during their period of growth they are 

 nucleated. 



The typical life-history of a cell has been epitomized in the statement 



1 Leydig, Lehrbuch der Histologie, 1857, S. 9. 



