22 THE INDIVIDUAL ORGANISM 



of each kind of plant or animal have a characteristic and fixed number of 

 chromosomes. In each case these chromosomes can be grouped into half 

 that number of identical pairs, regardless of the total number of chromo- 

 somes present. The characteristic number of chromosomes may range 

 from 2 to more than 100 in different species. 



TISSUES AND GLANDS 



A tissue may be either a group of like cells, so connected as to perform 

 a common function, or the common product of a group of similar cells. 

 In any of the higher animals, scores of different kinds of tissues may be 

 recognized, but for our purposes we can group all tissues into four great 

 groups : 



1. Sustentative tissues are adapted to provide mechanical support 

 and to bind various other tissues together. In addition, certain types of 

 sustentative tissues are modified for such special functions as fat storage 

 or the formation of a circulating medium. Examples of sustentative 

 tissues are bone, cartilage, ligament, tendon, and the cells and fluids of the 

 blood. Many of these tissues are chiefly cell products, their living cells 

 functioning to provide the needed material rather than to perform the 

 necessary tasks directly. 



2. Contractile tissues include various types of muscle cells; all are 

 adapted to perform work by contracting or shortening. 



3. Nervous tissues are specialized to receive stimuli and transmit 

 impulses. 



4. Epithelial tissues are variously specialized to cover the external 

 and internal surfaces of the body. The formation of glands is a charac- 

 teristic and peculiar property of epithelial tissues. 



Glands are cells or groups of cells that have the function of secreting 

 some substance needed in the economy of the body or of excreting some 

 waste product or surplus substance. In all glands, the actual secreting 

 (or excreting) cells are derived from epithelial tissues, although these may 

 be supplemented by various sustentative and contractile tissues. In 

 some instances, single epithelial cells (gland cells) function as glands, 

 but typically glands are aggregations of a number of secreting (or excret- 

 ing) cells that are able to produce a comparatively large quantity of their 

 particular product. 



Ordinarily, we think of a gland as being connected with some epi- 

 thelial surface (the skin or the lining of the gut) by a passageway or duct, 

 through which its products may pass. Examples of such glands will 

 be found in the skin and in the digestive glands of the mouth, stomach, 

 and intestine. Another very different kind of gland will be considered 

 when we come to study the endocrine system. 



