CELLS AND TISSUES. 7 



ferentiation of organs involves the division of physiological 

 labor. 



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

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

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

 organs develop originally from epithelium, 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 through the skin of an embryonic shark, showing at E the 

 epithelial cells, forming pavement epithelfum ; and at e, columnar epithelium. After 

 Gegenbaur. 



rian, as Codosiga, 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 filled with a ge- 

 latinous fluid or protoplasm, and occurs between muscles, 

 etc. This tissue forms in the vertebrates the " notochord " 

 i.e., a rod supporting the 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 are also varieties of connective tissue. 



Cartilaginous tissue is characterized by cells situated in a 



