LIVING SUBSTANCE 65 



been extended from year to year, and the conception of the cell 

 has been made constantly more precise. 



The conception of the nature of the cell has not been always the 

 same. As we have seen,i the cell idea originated as a result of 

 the microscopic observation of plants. The microscopists of the 

 seventeenth and eighteenth centuries found that plant-tissue 

 contained, besides long tube-like structures, small chamber-like 

 elements set off from one another by walls, and containing liquid. 

 Because of their similarity to the large cells of honeycomb these 

 small structures received the name of " cells." Thus, at that time 

 the cell was regarded as a simple droplet of liquid enclosed by a 

 wall or membrane. The characteristic thing which led to the 

 giving of the name " cell," a term very fitting for plant-cells, was 

 the " cell-membrane," without which a chamber, vesicle, or cell was 

 not possible. This idea continued to prevail even when Schleiden 

 discovered, in addition to the cell-liquid or cell-sac, a slimy semi- 

 liquid mass, the " plant-slime," or, as Mohl called it, the " proto- 

 plasm," and when by Schwann the cell idea was extended to the 

 elementary parts of animal tissues. 



The fundamental work of Max Schultze ('61, '63) gave to 

 the cell idea an entirely different meaning. The study of the 

 UMzcypoda, those one-celled organisms whose naked protoplasmic 

 bodies are capable of extending their viscous body-substance at any 

 desired spot into fine threads and networks, led Schultze to the 

 view that the essential part of the cell cannot be the cell- 

 membrane, for the very numerous species of Bhizopoda have 

 throughout life no cell-membrane ; but that it is the substance 

 which earlier had been termed "sarcode" by Dujardin ('41) in 

 naked fresh-water BMzopoda and Infusoria. A comparison of 

 Rhizopoda and plant-cells afforded Schultze the proof that sarcode, 

 the substance of the BMzopoda, is completely identical with proto- 

 plasm, the viscous contents of plant-cells ; and thus he founded 

 the theory of protoplasm, according to which the essential 

 constituent of the cell is the protoplasm. The idea that the cell 

 is a simple bit of protoplasm has proved brilliant in results, in 

 opposition to the old view of the necessity of the cell-membrane. 

 Not only have an enormous number of cells that lack a membrane 

 become known among the numerous unicellular Bhizopoda (to which 

 belong the Polythalamia or Foraniinifera having calcareous shells, 

 the Hadiolaria having silicious shells, and the Amcebw in which a 

 shell is wholly wanting), but it has also been observed that in the 

 development of many plants and animals one-celled stages occur 

 as eggs, which are entirely devoid of a membrane. Hence, since 

 Max Schultze's establishment of the protoplasm theory, the idea 

 that the cell-membrane is a general cell-constituent has completely 

 disappeared. 



' Cf. p. ■!-. 



