5O GENERAL SKETCH OF THE CELL 



Space would not suffice for a comparative account of the endless 

 modifications shown by the cytoplasmic substance in different forms 

 of cells. Many of these arise through special differentiations of the 

 active substance, the character of the structure thus being some- 

 times so highly modified, as in the striated muscle-fibre, that it is 

 difficult to trace its exact relation to the more usual forms. More 

 commonly the cytoplasm is modified through the formation of passive 

 or metaplasmic substances which often completely transform the 

 original appearance of the cell. The most frequent of such modifi- 

 cations arise through the deposit of liquid drops and " granules " 

 (many of the latter, however, being no doubt liquid in life). When 

 the liquid drops are of watery nature the cavities in which they lie 

 are known as vacuoles, which are especially characteristic of the pro- 

 toplasm of plant-cells and of Protozoa. These may enlarge or run 

 together to form extensive cavities in the cell, the protoplasm becom- 

 ing reduced to a peripheral layer, or to strands and networks travers- 

 ing the spaces ; while in some forms of unicellular glands the spaces 

 may form branching canals traversing the protoplasm. 



The vacuolization or meshlike appearance arising through the 

 formation of larger vacuoles or the deposit of other metaplasmic 

 material is not to be confounded with the primary protoplasmic struc- 

 ture. When, however, smaller vacuoles or metaplasmic granules are 

 evenly distributed through the protoplasm, a " pseudo-alveolar " struc- 

 ture (Reinke) arises that can often hardly be distinguished from the 

 "true" alveolar structure of Biitschli. 1 Comparative study shows 

 that all gradations exist between the "false " and the "true " alveolar 

 structures and that no logical ground of distinction between the two 

 exists. 2 We thus reach ground for the conclusion that the coarser 

 secondary alveolar or reticular formations are to be regarded as only 

 an exaggeration of the primary structure, and that the alveolar mate- 

 rial of Biitschli's structure belongs in the same general category with 

 the passive or metaplasmic substance. 3 



E. THE CENTROSOME 



The centrosome 4 is usually an extremely minute body, or more 

 commonly a pair of bodies, staining intensely with haematoxylin and 



1 In the latter the alveolar spheres are, according to Biitschli, not more than one or two 

 microns in diameter. 



2 This has been demonstrated in the cells of plants by Crato ('96), and more recently 

 by the writer ('99), in the case of echinoderm and other eggs. 



* Cf. p. 2 9 . 



4 The centrosome was apparently first seen and described by Flemming in 1875, in the 

 egg of the fresh-water mussel Anodonta, and independently discovered by Van Beneden, in 



