74 GENERAL PRINCIPLES OF ZOOLOGY. 



alone forms the egg in the narrower sense, apart from its 

 coverings, has for a time the morphological value of a cell, 

 and in the case of the ostrich egg may reach a diameter of 

 several inches. The form of the cell is likewise variable. 

 Free living cells, whose form is not determined by the en- 

 vironment, in the resting condition 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 run out spindle- or star-shaped 

 projections. 



Protoplasm. So there is left to characterize the cell 

 only its constituent substance, the protoplasm. Here, 

 also, is a distinction of a chemical character. It is not 

 known whether protoplasm is a definite chemical body, 

 which from its constitution is capable of infinite variation, 

 or whether it is a varying mixture of different chemical 

 substances. So, also, we are by no means certain whether 

 or not these substances (as one is inclined to believe) be- 

 long to those, in themselves, enigmatical substances the 

 proteids. We can only say that the constitution of proto- 

 plasm must, with a certain degree of homogeneity, have a 

 very extraordinary diversity. For if we see that from the 

 egg of a dog there comes always and only a dog, and in- 

 deed an animal with all his individual peculiarities, that a 

 sea-urchin's egg, placed under the most diverse condi- 

 tions, produces always a sea-urchin, that a species of 

 amoeba always performs only the movements characteristic 

 of that species, we must assume that the functioning con- 

 stituent part of this cell, the protoplasm, has in each case 

 its peculiarities. We are driven to the assumption of an 

 almost unlimited diversity of protoplasm, even if we con- 

 cede an important share in the prominent differences to the 

 nucleus, of which we shall speak later. 



General Properties of Protoplasm. The similarity of 

 protoplasm, still recognizable through all its variations, ex- 

 presses itself in its appearance and in its vital phenomena. 

 Under slight magnification, protoplasm appears as a faintly- 

 gray substance, sometimes colored yellowish, reddish, etc., 



