16 



ZOOLOGY 



SECT. 



their existence 9,11 animals are formed entirely of protoplasm. 

 Every animal consists at first of a single minute particle of proto- 

 plasm, not widely different from an Amoeba. Soon this particle 

 divides into a number of parts which, instead of separating 

 completely from one another, like the parts of a divided Arnceba, 

 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 into the 

 meaning of these two terms cell and protoplasm evidently so 

 important in the study of both plants and animals. Proto- 

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

 granular substance of complex chemical composition. It is known 



not to be a definite 

 compound, but to be a 

 somewhat varying mix- 

 ture of chemical com- 

 pounds, the most essential 

 of which are bodies of 

 the class of proteids 

 highly complex sub- 

 stances, into the composi- 

 tion of which the elements 

 carbon, hydrogen, oxygen, 

 nitrogen, and sulphur all 

 enter. Living protoplasm 

 always contains a large 

 amount of water. It is 

 capable in the living condi- 

 tion of passing from the viscid or semi-fluid condition into a 

 gelatinous one in the whole or in parts of a cell. It is soluble in 

 weak acids and 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, dis- 

 tinctly marked off from one another. One of these lands of material 

 is apparently of less fluid consistency than the other. According 

 to one view (alveolar theory) the two kinds are intimately com- 

 bined in the form of an emulsion or froth, the one forming the 

 minute vesicles or bubbles in the froth, the other the ground- 

 substance in which the bubbles are embedded (Fig. 3). Accord- 

 ing to another view (rcticular theory), one of these substances, 

 the less fluid, appears to be arranged in the form of a network of 

 threads, composed of numerous minute rounded granules enclosing 



FIG. 3. Diagram to illustrate the alveolar theory of 

 protoplasm. (After .Dahlgren and Kepuer.) 



