STRUCTURE AND PHYSIOLOGY IN ANIMALS 



15 



of organs — muscles, alimentary or enteric canal, glands, heart and 

 blood-vessels, gills or lungs, nervous system, organs of excretion, and 

 organs of reproduction. But in all animals, however complex, the 

 same substance, protoplasm, which in Amoeba constitutes the 

 bulk of the body, is the essential and active part. Wherever in 

 the body active functions are being discharged and active changes 

 are going on, there we find protoplasm present ; where there is 

 no protoplasm there is no vital activity. In the earliest stages of 

 their existence all 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 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 into 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 contains a large 

 a moun t of water. It is 

 soluble in weak acids or 

 weak alkalies ; and is 

 capable of being coagu- 

 lated — rendered firmer 

 and more opaque — by 

 the action of heat and 

 of strong alcohol. Its re- 

 action is slightly alkaline. 

 As regards its minute 

 structure, it is generally 

 acknowledged that there 

 are two kinds of sub- 

 stance in the protoplasm, in some cases more, in others less, dis- 

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

 is apparently of less fluid consistency than the other. According 

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



Fio. 3. — Diagram to Illustrate the alveolar theory of 

 protoplasm. (After Dahlgren and Kepner.) 



