24 



PRINCIPLES OF VETERINARY SCIENCE 



germ center is the essential and active part in reproduction. 

 The cell nucleolus is a minute rounded body contained in the nu- 

 cleus. The cell centre-some is a highly refractive body with many 

 fine radiating fibers and has to do with cell division. 



Fig. 2. — Different types of cells composing the body. A, Typical cell showing : 

 e, centrosome; /, food granules in the protoplasm; n, nucleus; M.S., nucleolus; 

 B, complete cell; C, nerve cell with axon and dendrites; D, involuntary muscle- 

 fiber; E, endothelial cells; F, ciliated epithelial cells from trachea; G, stratified 

 squamous epithelium from the skin. (Jordan and Kellogg, Evolution and Animal 

 Life.) 



Protoplasm is a transparent, viscid substance to the sight and 

 touch. On close examination it is found to be a complex mass 

 of organic material held together by a very delicate meshwork 

 of fibers. In plants and animals protoplasm is essentially the 

 same. It contains about 80 per cent of water, which is in a state 

 of chemical combination. 



Three well-known physical forces operate in living protoplasm. 

 They have been described by Wentworth as follows: First, 



Digitized by Microsoft® 



