GENERAL ANATOMY. 61 



Mycetozoa, are the egg cells. The yolk of the bird's egg, which 

 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 cells, whose form is not 

 determined by the environment, are usually spherical or oval in 

 the resting condition, 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 spindle- or star-shaped 

 branching processes. 



Protoplasm. So there is left to characterize the cell only the 

 constitution of its substance: the cell is a mass of protoplasm with 

 one or more nuclei. 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) belong to 

 those other enigmatical substances, the proteids. We can only 

 say that the constitution 01 protoplasm 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 indeed an animal with all his individual peculiarities, that a 

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

 duces always a sea-urchin, that a species of amoeba always performs 

 only the movements characteristic of that species, we must assume 

 that the functioning constituent 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 concede 

 an important share in the prominent differences to the nucleus, of 

 which we shall speak later. 



General Properties of Protoplasm. The similarity of proto- 

 plasm, still recognizable through all its variations, expresses itself 

 in its appearance and in its vital phenomena Under slight mag- 

 nification, protoplasm appears as a faintly-gray substance, some- 

 times colored yellowish, reddish, etc., by pigments taken up by 

 imbibition, in which numerous strongly-refracting granules are 

 embedded. The vital characteristics of this substance are move- 

 ment, irritability, power of assimilation and of reproduction. 



By using higher powers a finer structure can be seen in the 

 protoplasmic substance, the ' homogeneous protoplasm ' of earlier 

 writers. The nature of this is as yet in question : a fine-meshed 

 framework (filar substance, spongioplasm, cell reticulum) the 



