BASIC ORGANIZATION OF THE METAZOA 



83 



tural resemblance to some of the supporting tissues. It is a very loose 

 tissue whose cells are irregular, often star-shaped. These cells are not 



closely packed but touch one another 

 only by their corners or the tips of 

 their projections (Fig. 70). Con- 

 siderable space is thus left among the 



Fig. 69. Fig. 70. 



Fig. 69. — Section through bone, showing the stellate spaces in the matrix occupied by 

 cells, and at left part of the space occupied by a blood vessel. (From Hill, "Manual of 

 Histology and Organography,'' W. B. Saunders Company.) 



Fig. 70. — Mesenchyme from umbilical cord. {From Hill, "Manual of Histology and 

 Organography," W. B. Saunders Company.) 



Thi 



s 



cells, which is filled with some more or less liquid substance, 

 spongy structure is everywhere characteristic of mesenchyme. 



Epithelial Tissue. — An epithelium is a layer of cells covering some 

 surface, either the outside of an organ or the lining of the wall of a cavity. 



Fig. 71. — Types of epithelium. A, columnar; B, cubical; C and D, squamous (side 

 and surface views, respectively); E, ciliated; F, flagellate; G, collared; H, stratified; vac, 

 vacuole. 



The endoderm and ectoderm of Hydra, already described, are epithelia. 

 Others likely to be observed in laboratory studies are the outer layer 

 (hypodermis) of the body wall of the earthworm, the lining of the intestine 



