98 THE PROTOZOA 



chromatin does not form a distinct nucleus. In all Protozoa there 

 is a true nucleus in at least the principal stages of the. life-history, 

 and it is obvious that the recognition of a cellular grade, charac- 

 terized by the possession of a true nucleus, postulates that the first 

 origin and evolution of the nucleus must be sought amongst those 

 organisms which have been classed, speaking broadly, as the 

 bacterial grade. We may expect, therefore, to find in organisms 

 which stand on the plane of morphological differentiation which 

 characterizes the bacteria the early stages of the evolution of the 

 nucleus from the primitive chromidial condition, and even cases 

 in which the condition of a true nucleus has been reached. The 

 matter cannot be discussed further here, where it must suffice to 

 establish the existence of true nuclei in Protozoa ; but Dobell (52) 

 has described an interesting series of conditions which may be 

 regarded as stages in the evolution of nuclei amongst bacterial 

 organisms. 



Since the possession of a true nucleus has been regarded here 

 as the criterion of the cellular grade of organization, it is necessary 

 to discuss briefly the meaning and application of the term " cell." 

 By many, perhaps most modern writers, the cell has been regarded 

 as the elementary vital unit, than which there exists nothing 

 fcimpler amongst living beings. In this sense the word " cell " becomes 

 synonymous with the term " micro-organism," "protist," or any other 

 word used to denote living beings of the most primitive type : 

 " tout ce qui vit riest que cellules " (Delage and Herouard, 6). The 

 word " cell " was, however, applied originally to the elements that 

 built up the tissues of animals and plants. At first, as the word 

 cell implies, it was used to denote only the enclosing membrane or 

 framework ; but when it became apparent that the membrane was 

 of secondary importance, it was transferred to the contained stuff, 

 and so came to signify a structural element in which the living 

 substance, protoplasm, is differentiated into two distinct parts- 

 nucleus and cytoplasm. If the term " cell " is not to become so 

 vague and indefinite in its significance as to be absolutely meaning- 

 less, it is best to restrict its application to living organisms which 

 have reached this degree of differentiation. Dobell considers that 

 all Protista are nucleated organisms ; in the preceding paragraphs 

 reasons have been advanced against accepting this proposition as 

 a statement of fact, and from the point of view of phylogenetic 

 speculation, I, at least, find it difficult to believe that the earliest 

 form of life could have been an organism "in which the living sub- 

 stance was differentiated ab initio into distinct nucleus and cyto- 

 plasm. 



In my opinion the cell, as defined above that is to say, an 

 organism in which the living substance, protoplasm, has become 



