LIVING SUBSTANCE 23 



expressed in millimeters. The latter are commonly observed to possess 

 ameboid motion. Consequently, the formation of a bulky organism 

 is possible only by the union of a multitude of relatively independent 

 cellular elements. 



As has been stated above, the term protoplasm was employed origi- 

 nally in a morphological sense to designate the entire mass of living 

 substance inside the cell wall with the exception of the nucleus. At 

 the present time, however, we know that this conception is not quite 

 correct, because the contents of the cell are really a morphological and 

 chemical mixture. To begin with, it may be stated that a cell con- 

 sists of two parts, namely of cytoplasm and of nuclear material. 



The cytoplasm appears as a clear homogeneous, viscous *' ground- 

 substance" in which are embedded varying numbers of formed ele- 

 ments.^ At times, therefore, the watery ground substance is clearly 

 in evidence, while at other times it is hidden by granular material. 

 The formed elements of the cytoplasm embrace bodies which are abso- 

 lutely essential to the life of the cell as well as bodies which must be 

 regarded as accidental admixtures. Among the former are granules 

 representing all stages of metabolism, namely, food material ready 

 for assimilation and the products of the cellular processes ready for ex- 

 cretion. Some of the latter may first be transported to distant parts 

 of the body to be used in connection with some other function. A very 

 important constituent of the cytoplasm of the green plants is the 

 so-called chloroplastic material which appears as small round or tape- 

 shaped bodies containing an intense green pigment. It is the func- 

 tion of this material to assimilate the carbon dioxid so that under the 

 energy of the rays of the sun an assimilation of starch and an evolu- 

 tion of oxygen may be had. A similar substance is the leukoplastic 

 material of certain plant cells which serves to build up starch from 

 sugars. At times the cytoplasm also contains globules of fluids, the 

 so-called vacuoles, which may be either quiescent or exhibit rhythmic 

 contractions. Among the accidental admixtures may be mentioned 

 the indigestible remnants of the food, such as pieces of the shells, 

 skeletons or capsules of the organisms which have been ingested. In 

 fact, the cytoplasm may also give lodgment to living organisms and 

 especially to certain parasites. 



Under the low power of the microscope the ground-substance of the 

 cytoplasm presents a perfectly homogeneous hyaline appearance; 

 indeed, such objects as the pseudopodia of the ameba and rhizo- 

 pods do not display a differentiation even when observed under high 

 powers. In most cases, however, some kind of structure may then 

 be made out. Thus, Remak^ has shown (1844) that ganglion cells 

 possess a fibrillar interior, while Frommann and Heitzmann have 

 proved (1867) that the fundamental structure of protoplasm is spongy. 



1 M. Heidenhain, Plasma und Zelle, Jena, 1911. 



2 Archiv ftir Anat. und Physiol., 1844. 



