56 INTRODUCTION 



same microscopical structures as are shown by the cytoplasm 

 of cells. Network, foam, and alveolar structures are produced 

 in albumin and gelatin solutions when they are hardened by 

 bichloride of mercury, osmic acid, formalin, etc., and the same 

 characteristic differences that are produced in cells by these dif- 

 ferent reagents are likewise produced in the hardened proteid 

 solution, Proteid structures hardened under strain form radi- 

 ating structures resembling centrosomes and the radiating threads 

 seen in cells. If elder pith is saturated with proteid solutions 

 and then hardened, sectioned, and stained by the usual methods, 

 appearances resembling closely the structure of a hardened cell 

 may be found in the spaces of the pith even a central, nucleus- 

 like mass may be suspended in a network of anastomosing 

 threads. These and many other experiments indicate that much 

 of the work done on cell structure by means of studies of hard- 

 ened cells cannot be considered of value in deciding the struc- 

 ture of living cells ; but, nevertheless, the fact remains that 

 many cells that can be observed while alive and uninjured under 

 the microscope are seen to have a definite structure in the cyto- 

 plasm, e. g., sea-urchin eggs, which show a characteristic alve- 

 olar structure. 



A compromise view of the structure of protoplasm (and 

 cytoplasm in particular) which takes account of what appear to 

 be facts brought out on both sides of the question, is that while 

 in some cells definite structural arrangements of the cytoplasm 

 exist, in most cells, and to a large extent even in cells showing 

 a cytoplasmic structure, the proteids are in a homogeneous 

 solution ; most of the structures seen in fixed cells, except the 

 chromatin threads, nuclear membrane, nucleoli, and centrosomes, 

 are produced by the coagulation of the proteids, and are not 

 present during life. When a framework does exist, it is a fair 

 inference, by analogy with the cell membrane and the stroma 

 of the red corpuscles, that the cell lipoids are largely responsible 

 for its formation, and that they form a prominent part of its 

 composition. This question of the presence or absence of struc- 

 ture in the cytoplasm is of more interest than as a mere mor- 

 phological problem, for if the cytoplasm is subdivided into 

 innumerable little chambers, each surrounded by a membrane, 

 it is probable that processes of diffusion and conditions of 

 osmotic pressure will be very different from what they would be 

 if the cytoplasm were a simple homogeneous colloid solution, like 

 a lump of semisolid gelatin or agar. In such colloidal masses 

 diffusion and osmosis go on almost as if there were no colloids 

 in the solvent at all, whereas most membrane structures that 



