CELL STRUCTURE 43 



l)ith — even a (-(Mitral, iiuclcus-likc mass may be suspencJcd in a net- 

 work of anastomosing threads. These and many other experi- 

 ments indicate that much of tlie work done on cell structure by means 

 of studies of hardened cells cannot be considered of value in deciding 

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

 man}^ cells that can be observed while alive and uninjured under the 

 microscope are seen to have a definite structure in the cytoplasm, 

 e. g., sea-urchin eggs, which show a characteristic alveolar 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 the 

 proteins are chiefly in a homogeneous solution; most of the structures 

 seen in fixed cells, except the mitochondria, chromatin threads, nuclear 

 membrane, nucleoli, and centrosomes, are produced by the coagulation 

 of the proteins, and are not present during life. When a framework 

 does exist, it is a fair inference, by analogy with the cell mem- 

 brane 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 

 structure in the cytoplasm is of more importance than as a mere mor- 

 phological problem, for if the cytoplasm is subdivided into innumer- 

 able 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 struc- 

 tures that are found in living tissues seem to have a decidedly semi- 

 permeable character. 



From what we know at the present time of intracellular physics 

 and chemistry there is no necessity for assuming that semipermeable 

 septa exist within the cell. All the intracellular processes with which 

 we are familiar could go on without such structures. It is not neces- 

 sary to assume a compartment structure to explain the possibility of 

 different chemical reactions going on in different parts of the cell at 

 the same time, for most of the cell reactions seem to depend on 

 enzymes, which we know are not readily diffusible in solutions of col- 

 loids, and, therefore, might remain fixed without requiring any en- 

 closing walls or retaining framework. Certainly, many cells are free 

 from structural cytoplasm, for we see particles of solid matter moving 

 about within them quite freely. In some cells the nuclei migrate 

 about in the cell, as also do digestive and excretory vacuoles, which 

 motion would seem to be rather destructive if the protoplasm had a 

 structure at all permanent. 



When a portion of the cytoplasm is cut free from the body of 



