CHAPTER IV 



The cell theory Unicellular organisms Differentiation and division of 

 labour Co-operation The transition from the unicellular to the 

 multicellular condition The early development of multicellular animals 

 and plants. 



WE have seen that, although Haematococcus and Amoeba differ 

 widely from one another in various respects, they nevertheless 

 exhibit a fundamental agreement in structure, for each consists 

 essentially of a single nucleated mass of protoplasm, each is a 

 single cell. 



The history of the term cell is a curious one, and affords a 

 good illustration of the manner in which our scientific conceptions 



FIG. 6. Thin Section of a Bottle Cork, showing cellular Structure, X 170. 

 (From a photograph.) 



gradually become modified and improved as our knowledge 

 increases. Nothing was known of cells before the invention of 

 the microscope, but in the latter half of the seventeenth century 

 this invention opened up an entirely new field of research, and 

 enabled the earlier microscopists to lay the foundations of the 

 modern science of biology. To Eobert Hooke has been assigned the 

 credit of first observing the cellular structure of vegetable tissues, 

 and his observations, published in 1665, were soon afterwards 



