i STRUCTURE AND PHYSIOLOGY OF ANIMALS 15 



separating completely from one another, like the parts of a 

 divided Amoeba, remain associated together, forming a clump 

 of minute particles of protoplasm. Such minute protoplasmic 

 particles are termed cells ; every animal consists, at first, of a 

 single cell, and afterwards, in all higher animals, this single cell 

 becomes converted by division and subdivision into a little cluster 

 or clump of cells. 



It is time that we should inquire more particularly as to the 

 meaning of these two terms cell and protoplasm evidently so 

 important in the study of both plants and animals. Protoplasm, 

 we have already seen, is a semi-fluid, gelatinous, clear or finely 

 granular substance of complex chemical composition. It is known 

 not to be a definite compound, but to be a somewhat varying 

 mixture of chemical compounds, the most essential of which are 

 bodies of the class of proteids highly complex substances, into the 

 composition of which the elements carbon, hydrogen, oxygen, 

 nitrogen, and sulphur all enter. Living protoplasm always con- 

 tains a large amount of water. It is soluble in weak acids or 

 weak alkalies ; and is capable of being coagulated rendered firmer 

 and more opaque by the action of heat and of strong alcohol. Its 

 reaction is slightly alkaline. As regards its minute structure, it 

 is generally acknowledged that there are two kinds of substance in. 

 the protoplasm, in some cases more, in others less, distinctly marked 

 off from one another. One of these substances (mitome) is less 

 fluid than the other, and appears to be arranged in the form of a 

 network of threads, composed of numerous minute rounded gran- 

 ules, enclosing the second, more fluid substance {paramitomc} in 

 its meshes. 



To a particle of protoplasm, usually containing a nucleus in its 

 interior, constituting the entire body of such a simple organism as 

 Amoeba, and forming one of the constituent elements of which a 

 higher plant or animal is made up, the term cell is applied. The 

 word was first employed in reference to the microscopic struc- 

 ture of plants, in connection with which it is much more appro- 

 priate than in connection with the microscopic structure of animals ; 

 for a plant-cell has, nearly always, a definite, firm, enclosing 

 envelope or cdl-wcdl (Fig. 3, I, c.w) a structure which is only 

 exceptionally present in the case of animals. In the interior 

 of the cell-protoplasm, or cytoplasm, is a body termed the nude it* 

 similar to the nucleus of Amoeba ; usually of rounded shape, with 

 a thin enclosing nuclear membrane (A, nu.m), which is perforated 

 by numerous minute apertures. In the nucleus is a single coiled 

 thread, or a network of threads, or one or more rounded clumps, 

 of a substance chromatin (chr.) which differs from ordinary 

 protoplasm in having a stronger affinity for most staining 

 agents. A rounded body termed the nudeolus (nu), which usually 

 occurs in the interior of the nucleus, is formed either of a 



