ANIMAL TISSUES. 113 



CHAPTER X. 



ANIMAL ELEMENTS AND TISSUES. 



THE tissues of which animals consist, like those of 

 plants, are primarily derived from cells ; in fact the 

 essential part of the egg or ovum, from which all per- 

 fect animals originate consists at first only of a simple 

 cell, with its nucleus and nucleolus. 



The animal cell- wall differs from that of the vege- 

 table cell in its softness and delicacy also in its che- 

 mical composition, the former consisting of albu- 

 minous (albumen, white of egg) matter, while the 

 latter is composed of cellular or vegetable-cell sub- 

 stance. 



There is also a striking difference between vegetable 

 and animal tissues, in the circumstance that, while 

 the former retain their cellular condition to a very 

 great extent, the cells of the latter are frequently so 

 altered by compression and fusion together, or are 

 obscured by the great development of the cell-con- 

 tents, that the cell-form is obliterated, or can only be 

 discovered by the application of chemical reagents ; 

 and in many instances, the relation of the tissues to 

 the cell can only be discovered by tracing the growth 

 or development of the latter from its earliest stages. 

 Hence the examination of the elements and tissues 

 of animals is not well adapted for those who are un- 

 practised in the use of the microscope ; and in treat- 

 ing of them, we shall simply notice a few which are 

 most easily examined, beginning with those found 

 in animals belonging to the subkingdom Vertebrata 

 (ver'tebra, a spine-bone) . 



MAMMALIA. The animals belonging to this class 



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