74 GENERAL PRINCIPLES OF ZOOLOGY 



the sensory cell, the connection being one of contact, not of continuity. In 

 both the peripheral end of the cell bears appendages for sense perception; 

 auditory and tactile hairs, stronger processes in olfactory and taste 

 cells, conspicuous rods in visual cells. Almost without exception the 

 sensory cells are part of the skin (ectoderm), or arise from it in develop- 

 ment. This is true for sense organs like the eye and ear of vertebrates, 

 which are separated from the skin by thick intermediate tissue, for in these 

 the sensory epithelium (retina, crista acustica) is derived from the ecto_- 

 derm. Recent studies seem to show that the taste organs in some forms 

 are entodermal in origin. 



Supporting Cells. In sensory epithelium between the sensory cells 

 are found still other epithelial cells, which are not connected with nerves, 

 but have accessory functions: they serve as supports for the sensory cells; 

 in the eyes they contain pigment; in the auditory organs they often bear 

 the otoliths, etc. They have the general name of supporting cells. 



2. Supporting or Connective Tissues. 



From a histological point of view there is no greater difference than 

 that between epithelium and connective tissue; the former belongs to the 

 surface, the latter to the interior of the body; in the former the cells play 

 the chief role, in the latter their importance is subordinate to the plasmic 

 products, the intercellular substances which chiefly determine the character 

 of the various kinds of connective tissue. 



In spite of this contrast the connective tissues are genetically connected 

 with epithelium. In embryos, which at first consist only of epithelia, 

 the connection can be directly seen. The epithelia secrete a gelatinous 

 substance from their deeper surfaces into which separate cells migrate. 

 Thus arises the embryonic connective tissue, the mesenchyme (fig. 108). 



Function of Connective Tissue. The primary function of con- 

 nective tissue is to fill the spaces between the various organs in the 

 interior of the body, thus connecting not only the single parts of the organs, 

 but also the various organs themselves. In consequence of this the con- 

 nective tissues contribute to the firmness of body, and are frequently 

 employed in building up a skeleton. To accomplish this, substances which 

 are usually firmer than protoplasm are formed on the surface of the cells, 

 and, since they lie between the cells, these are called intercellular substances. 

 In proportion as the intercellular substance increases in volume the cells 

 themselves diminish and become inconspicuous connective-tissue cor- 

 puscles, or, as seldom happens, entirely disappear. Since in connective 

 tissues, the intercellular substances are most important, it follows that the 



