THE FROG a3 
portion that shows clearly as many of the above features as 
possible. Be sure to include a mucus gland, if possible one show- 
ing the neck extending through the epidermis. 
XII. EpirHecium. 
An example of flattened or squamous epithelium is furnished 
by the stratum corneum of the integument. Simple columnar 
epithelium is found in the mucosa of the intestine. Ciliated 
epithelium is found lining the cesophagus and the roof of the 
mouth of the frog, and may be demonstrated as follows: 
Gently scrape the roof of the mouth of a recently-killed frog, 
transfer the scrapings to a slide and mount in mucus from the 
frog’s mouth or in normal salt solution. Examine with the 
high power of the microscope. At the free edges of some of 
the cells a shimmering movement will be seen; this is often so 
rapid that it is impossible to see the cilia which cause it, but after 
the movement has slackened somewhat it can be seen to be due to 
delicate, hair-like processes of the cells. The stroke of a cilium 
is always stronger in one direction and all the cilia beat in the 
same direction. Draw some of the cells and their cilia. 
Ciliated epithelium may also be seen by removing a small piece 
of the peritoneum supporting the oviduct and mounting it in 
normal salt solution. An excellent example of ciliated epithelium 
is afforded by the gills of the fresh-water mussel; mount a small 
piece (a single layer) of the gill in water. 
The following demonstration will be prepared by an instructor: 
Place a recently-killed frog on its back, cut away the lower jaw 
and the floor of the mouth in order to expose the roof of the 
mouth, and place some powdered carmine or bits of cork on the 
roof of the mouth. Observe that the particles are carried slowly 
back toward the cesophagus ; how? 
XIII. THe SKELETON. 
The skeleton, which forms the hard internal parts of the frog, 
is composed partly of cartilage and partly of bone. In the early 
stages of development the skeleton consists entirely of cartilage; 
in the adult this primary cartilaginous skeleton is replaced to a 
greater or less extent by bone. Bones formed thus by the replace- 
