r CONNECTIVE TISSUE 41 



smell, or taste. The sensory cells will engage our attention again 

 later. 



Epithelial cells may become pigment cells by the deposition in 

 their protoplasm of pigment material ; these pigment cells are an 

 important element in the external colouring of the animal. Deposited 

 in certain sensory cells, pigment serves for the absorption of light and 

 heat rays, and so contributes to the sensations of light, colour, and 

 warmth. 



Certain epithelial cells often perform the function of supporting 

 their companions. They form, by mingling with each other, a tissue 

 with meshes, the interstitial tissue, which is related to the other 

 epithelial cells as the mortar in a wall is related to the bricks. 



Since the protoplasm of neighbouring epithelial tissue mingles, 

 and since in epithelial cells it is the nuclei only and not their proto- 

 plasm that divide, masses of protoplasm with nuclei scattered through- 

 out them may arise. In such masses no cell boundaries can be recog- 

 nised. They are called syneytia. 



The epithelial cells do not always lie close to each other; they 

 are sometimes separated by clear intercellular spaces for transmission 

 of fluid, or by intercellular substances ; but in such cases they remain 

 connected by means of very fine protoplasmic processes which stretch 

 across these intercellular spaces. 



II. Connective Tissue. 



Under this name is comprised a long series of tissue forms, which 

 may have very different ^ 



origins, structures, and func- n.--.| V "'"[" iVi'Ai \ i . 

 tions. They originate either iM 

 directly or indirectly from 

 the epithelium. Their essen- 

 tial office is to bind together 

 different portions of the 

 body and different organs, 

 or to serve as support for 

 these by their possession of 

 a certain degree of firmness. 

 We divide the connective 

 tissues, according to an im- 

 portant difference in their : J|| 

 origin, into two principal PPI | 



groups. ef \ g 



I. GelatinOUS tiSSUe (Fig. FIG. S6. Gelatinous tissue of a Scyphomedusa (diagram- 

 36) takes its Origin direct matic).e, Epithelium ; jjr, Jelly; fy, epithelial cell passing in- 

 from an epithelium. To * ^jdly; 60, branched cells in the jelly; e /, elastic fibres. 



this belong, e.g., the gelatinous tissue of Medusae and the Ctenophwa. 

 Between the inner epithelium which lines the intestine and the 



