52 GENERAL PRINCIPLES OF ZOOLOGY 



algae, it, together with the chlorophyl, could leave the cell wall and swim 

 freely in the water, eventually giving rise to a new plant, while the cell 

 wall no longer showed signs of life. Then it was discovered that many 

 animal cells had no cell membrane. These observations at once placed 

 the cell wall in the background, while the protoplasm was recognized as 

 all important. Here, too, should be mentioned the researches on the Pro- 

 tozoa, by which it was recognised that these organisms had no true organs, 

 but carried on all of the functions of life by means of a granular substance, 

 at first called sarcode. 



Then followed the recognition of the identity of the protoplasm of 

 plants, the sarcode of the protozoa and the cell substance of animal cells. 

 Equally important was the new idea as to the modifications of cells 

 and the differentiation of tissues. These are not so much modifications 

 of form and the like, based on osmosis and other physical phenomena 

 as upon chemical changes. By means of its formative potentiality the 

 protoplasm gives rise to non-protoplasmic structural parts, as, for ex- 

 ample, -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. 



Nature of the Cell. The size of the animal cell varies; the smallest 

 are the male sexual cells, the spermatozoa, whose bodies, in case of the 

 mammals, may measure only 0.003 mm.; the largest, with the exception 

 of the giant plasmodia of some Mycetozoa, are the egg cells. The yolk 

 of the bird's egg, which alone forms the egg in the narrower sense, has for 

 a time the value of a cell, and in the ostrich egg may reach a diameter 

 of several inches. The form is likewise variable. Free cells, are usually 

 spherical or oval as the egg cell shows; united into tissues, the cells, on the 

 contrary, may be pressed together into polygonal or prismatic bodies, or 

 may send out branching processes. 



Protoplasm. So there is left to characterize the cell only its sub- 

 stance: the cell is a mass of protoplasm with one or more nuclei. It is 

 not known whether protoplasm is a definite chemical body, capable of 

 infinite variation, or is a varying mixture of different chemical substances. 

 So, also, we are not certain whether or not these substances (as one is 

 inclined to believe) belong to the proteids. We can only say that the con- 

 stitution of protoplasm must, with a certain degree of homogeneity, have 

 a very extraordinary diversity. For if from the egg of a dog there comes 

 always and only a dog with all his individual peculiarities; that a sea- 

 urchin's egg, under the most diverse conditions, produces always a sea- 



