42 



GENERAL SKETCH OF THE CELL 



protoplasm of relatively slight differentiation, while the more highly 

 differentiated products of cell-activity are laid down in the more 

 peripheral region of the cell, either in the cortical zone or at one 

 end of the cell.i -p^ig f^ct is full of meaning, not only because it is 

 an expression of the adaptation of the cell to its external environment, 

 but also because of its bearing on the problems of nutrition. ^ For if, 

 as we shall see reason to conclude in Chapter VI I., the nucleus be 

 immediately concerned with synthetic metabolism, we should expect 

 to find the immediate and less differentiated products of its action in 

 its neighbourhood, and on the whole the facts bear out this view. 



The most pressing of all questions regarding the cytoplasmic 

 structure is whether the sponge-like, fibrillar, or alveolar appearance 

 is a normal condition existing during life. There are many cases, 

 especially among plant-cells, in which the most careful examination 

 has thus far failed to reveal the presence of a reticulum, the cyto- 

 plasm appearing, even under the highest powers and after the most 

 careful treatment, merely as a finely granular substance. This and 

 the additional fact that the cytoplasm may show active streaming and 

 flowing movements, has led some authors, especially among bota- 

 nists, to regard the reticulum as non-essential and as being, when 

 present, either a secondary differentiation of the cytoplasmic sub- 

 stance specially developed for the performance of particular functions 

 or a mere coagulation-product due to the action of fixatives. It has 

 been shown that structureless proteids, such as egg-albumin and 

 other substances, when coagulated by various reagents, often show a 

 structure closely similar to that of protoplasm as observed in micro- 

 scopical sections. Flemming ('82) long since called attention to the 

 danger of mistaking such coagulation-products for normal structures 

 as seen in fixed and stained material, and his warning has been 

 emphasized by the later experiments of Berthold ('86), Schwarz ('87), 

 and especially of Biitschli ('92, '98), Fischer ('94, '95, '99), and 

 Hardy ('99). Butschli's extensive studies of such coagulation-phe- 

 nomena show that coagulated or dried albumin, starch-solutions, gela- 

 tin, gum arable, and other substances show a fine alveolar structure 

 scarcely to be distinguished from that which he believes to be the 

 normal and typical structure of protoplasm. Fischer and Hardy 

 have likewise made extensive tests of solutions of albumin, peptone, 

 and related substances, in various degrees of concentration, fixed and 

 stained by a great variety of the reagents ordinarily used for the 

 demonstration of cell-structures. The result was to produce a mar- 

 vellously close simulacrum of the appearances observed in the cell, 

 alveolar, reticulated, and fibrillar structures being produced that often 

 contain granules closely similar in every respect to those described as 



1 (7: p. 55. 2 See Kupffer ('90), pp. 473-476- 



