No. 590] 



THE EVOLUTION OF THE CELL 



117 



impossible that I should do more than attempt to indicate 

 in the most summary manner the various modifications 

 of the cell-type of organism, since to deal with them con- 

 scientiously would require a treatise rather than an ad- 

 dress, and, moreover, many such treatises exist already. 

 The most conspicuous modifications of cell-structure are 

 those affecting the periplasm, or, as we may now term it, 

 the cytoplasm. In the first place, the cell as a whole takes 

 various forms; primitively a little naked mass of pro- 

 toplasm tending to assume a spherical form under the 

 action of surface-tension when at rest, the cell-body may 

 acquire the most diverse specific forms maintained either 

 by the production of envelopes or various kinds of exo- 

 skeletal formations on the exterior of the protoplasmic 

 body, or of supporting endoskeletal structures formed in 

 the interior. The simple amoeboid streaming movements 

 become highly modified in various ways or replaced by 

 special locomotor mechanisms or organs, flagella, cilia, 

 etc., of various kinds. The internal alveolar cytoplasm 

 develops fibrillae and other structures of the most varied 

 nature and function, contractile, skeletal, nervous, and so 

 forth. The vacuole-system may be amplified and differ- 

 entiated in various ways and the cytoplasm acquires man- 

 ifold powers of internal or external secretion. And finally 

 the cytoplasm contains enclosures of the most varied kind, 

 some of them metaplastic products of the anabolic or 

 catabolic activity essential to the maintenance of life, 

 others of the nature of special cell-organs performing 

 definite functions, such as centrosomes, plastids, chro- 

 matophores, etc., of various kinds. 



With all the diverse modifications of the cytoplasmic 

 cell-body the nucleus remains comparatively uniform. It 

 m ay indeed vary infinitely in details of structure, but in 

 principle it remains a concentration or aggregation of nu- 

 merous grains of chromatin supported on some sort of 

 framework over which the grains are scattered or 

 clumped in various ways, supplemented usually by plastin 

 °r nucleolar substance either as a cementing ground-sub- 



