No. 593] THE EVOLUTION OF THE CELL 



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as is popularly supposed, but is one which did not come 

 into being until the evolution of organisms had reached 

 a relatively advanced stage, that, namely, of the true 

 nucleated cell. 



I have confined myself in this address to the evolution 

 of the cell as this organism is seen in its typical form in 

 the bodies of the multicellular organisms, starting from 

 the simplest conceivable type of living being, so far as 

 present knowledge enables us to conceive it. But there 

 is not the slightest reason to suppose that the evolution 

 of the Protista took place only in the direction of the 

 typical cell of the cytologist. Besides the main current 

 leading up to the typical cell there were certainly other 

 currents tending in other directions and leading to types 

 of structure very unlike the cells composing the bodies of 

 multicellular organisms. It is impossible that I should 

 do more here than indicate some of the divergent lines of 

 evolution, and I will confine myself to those seen in the 

 Protozoa. 



Taking as the starting-point and simplest condition in 

 the Protozoa a simple cell or protocyte, in which the body 

 consists of a small mass of cytoplasm containing a nu- 

 cleus, with or without chromidia in addition, an early 

 specialization of this must have been what I may term 

 the plasmodial condition, typical of Khizopods in which 

 the cv toplasm increased enormously to form relatively 

 large masses. The nucleus meanwhile either remains 

 single and grows very large or, more usually, a great 

 number of nuclei of moderate size are formed. From 

 this large plasmodial type is to be derived the forami- 

 niferal type, characterized by the creeping habit of life, 

 and probably also the radiolarian type, specialized for 

 the floating pelagic habit. Both foraminiferal and radio- 

 larian types are characterized by an excessive develop- 

 ment and elaboration of skeletal structures, and the geo- 

 logical record proves that these two types of organisms 

 attained to a high degree of specialization and diversity 



