e 
CELLS AND TISSUES. ? 
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 isthe 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 H the 
pt cells, forming the epidermis; c, corium; é, columnar epithelium.—Aftez 
egenbaur, 
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, 
ete. An analogous (but hypoblastic) tissue forms the “ no- 
tocord,” a rod supporting the bodies of vertebrate embryos. 
Gelatinous tissue is a variety of connective tissue found in 
the umbrella of jelly-fishes (Aurelia, ctc.). Fibrous and 
elastic tissue ave also varieties of connective tissue. 
Cartilaginous tissue is characterized by cells situated in a 
