CELLS AND TISSUES. 7 



ferentiation of organs involves the division of pliysiological 

 labor. 



Tissues. — Of the different kinds of tissues there is, first, 

 epithelial tissue (Fig. 4) consisting of cells with a nncleiis and 

 nucleolus, and placed side by side, forming a layer. All the 

 organs develop originally from e^i^7i«ZM««, which is the prim- 

 itive cell-structure and forms the tissues of the germ-layers. 

 Epithelial cells form the skin of animals, and also the lining 

 of the digestive canal. The cells of the latter may, as in 

 sponges, bear a general resemblance to a flagellate infuso- 



Fig. 4. — Vertical section tlirough thie skin of an embryonic sliark, eliowing at E the 

 epitlielial cells, forming the epidermis; c, corium; e, columnar epithelium. — Afte. 

 Gegenbaur. 



rian, as Oodosiga, or they may each bear many hairs, called 

 cilia, which by their constant motion maintain currents of 

 the fluids passing over the surface of the epithelium. The 

 tissue forming glands is simply modified epithelium. 



Connective tissue is formed by isolated rounded or elon- 

 gated cells with wide spaces between them tilled with a ge- 

 latinous fiuid or protoplasm, and occurs between muscles, 

 etc. An analogous (but hypoblastic) tissue forms the "no- 

 tocord," a rod supporting tlie bodies of vertebrate embryos. 

 Gelatinous tissue is a variety of connective tissue found in 

 the umbrella of jelly-fishes {Aurelia, etc.). Fibrous and 

 elastic tissue ai'e also varieties of connective tissue. 



Cartilaginous tissue is cliar.icterized by cells situated in a. 



