CELLULAR DIFFERENTIATION 53 



capable of being stimulated by contact or other stimuli to which the organism may 

 be exposed. Likewise in higher forms we find highly specialized areas of sensitive 

 cells, which can be shown to belong primarily to the epithelium. These are the 

 end organs of special sense, as touch, sight, and the like, and they get their special 

 value from their connection with what will be described presently as the nervous 

 tissues of the central nervous system. The sensory cells are typically spindle-like 

 or even hair-like in form, often extended as fine fibres at the inner end, whereby 

 connection is established with the nerves (Fig. 19, A). 



72. Reproductive Epithelium. The sexual cells, both male and female, arise 

 from epithelium, ectodermal, entodermal, or, as is usually the case, mesodermal. 

 The budding of the sexual epithelium, in the development of the germ cells, sug- 

 gests the formation of glands (Figs. 20, 21). The sexual cells often develop at the 

 expense of the epithelial and other cells about them. 



73. Supportive or Connective Tissues. This class of tissues 

 embraces the bulk of the non-active tissues in animals. They 

 vary much in appearance and structure, agreeing in little except 

 in their mesodermic origin, their passivity, and in the preva- 



FlG. 22. 



FIG. 22. Cellular Connective Tissue, showing large vacuoles, v, in the protoplasm. 



Questions on the figure. Would you say that these cells are of a high or a low 

 order of differentiation? Why? Is there any intercellular substance? Where is 

 tissue of this kind found? (See reference texts.) 



lence of intercellular substance. The intercellular substance 

 gives the distinctive character to the connective tissues, the 

 cells having a relatively unimportant place after the produc- 

 tion of the intercellular substance. The general function 

 of the supportive tissues is to bind and sustain the more active 

 tissues in their relations to the body as a whole. The classifica- 

 tion of supportive tissues is based on differences in the intercellu- 

 lar substance. This may be fluid (as in blood) or solid (as in 

 bone) ; it may be homogeneous (as in some forms of cartilage) , or 



