60 GENERAL PRINCIPLES OF ZOOLOGY. 



was not active, but rather that they are moved by a homogeneous sub- 

 stance in which they are embedded. This substance, which Mohl, in 

 order to bring it into prominence, named protoplasma. became by other 

 studies still more important. In the reproduction of the simplest algae, it 

 was found that the protoplasm, together with the chlorophyl bodies, col- 

 lected itself into an oval mass, and that this body left the cell membrane 

 and swam freely in the water. Since the cell-wall no longer showed signs 

 of life, while on the other hand the protoplasmic body came to rest and 

 formed a new plant, it was shown beyond doubt that this was the most 

 important constituent part of the cell (comp. fig. 115). 



2. In the study of animal tissues the importance of the peculiar cell- 

 substance, the protoplasm, was still more plainly brought out. Here, in 

 spite of the long-prevailing preconceived idea, unbiassed observation led to 

 the discovery that most animal cells had no cell-membrane. 



3. Very important, finally, was the study of the lowest organisms, the 

 Protozoa. Bujardin sought by extremely careful observations to prove 

 that these animals had no organs, but consisted of a uniform granular sub- 

 stance, the sarcode. The sarcode alone could produce all the vital phe- 

 nomena, such as movement, sensation, assimilation, previously ascribed to 

 many organs. Dujardin's theory was stoutly contested by Ehrenberg and 

 his school, but finally attained general acceptance through the epoch- 

 making work of Max Schultze and Haeckel. 



Schultze's Protoplasm Theory. On the basis of these three series of 

 observations, Max Schultze finally established the reformation of the cell 

 theory briefly sketched above, when by accurate study of the appearance 

 and the vital phenomena, and by means of numerous experiments, he 

 proved that the cell-substance of animals, the sarcode of Protozoa, and 

 the protoplasm of plants are identical, and that to this substance, for 

 which he retained the name protoplasm, all the vital phenomena of animals 

 and plants are referable in the ultimate analysis. The second important 

 modification concerns the changes of cells into tissues. These follow not 

 so much through changes of form and modification of the cells into the 

 tissue elements, as Schwann thought, but rather by means of chemical 

 changes. By means of its formative potentiality the protoplasm gives rise 

 to non-protoplasmic structural parts, as, for example, connective-tissue 

 fibrils, muscle fibrils, nerve fibres, etc. These give the various tissues their 

 specific character and perform their functions. The tissues also retain as 

 the source of life and formation the unemployed remnants of cells, the 

 connective-tissue corpuscles, muscle corpuscles, etc. We will now trace 

 out farther these two fundamental ideas in Max Schultze's ' protoplasm 

 theory,' and thereby briefly sketch the elements of the modern theory of 

 tissues. 



Nature of the Cell. The size of the animal cell varies to a 

 considerable degree; the smallest elements are the male sexual 

 cells, the spermatozoa, whose bodies, particularly in case of the 

 mammals, are even less than 0. 003 mm. ; the largest, on the other 

 hand, with the exception of the giant plasmodia of some 



