FOR WAGGONER’S BIOLOGY 
81 
of the intestine. (For a study of the nervous system a specially 
prepared dissection may be provided by the instructor.) The 
brain lies in a cavity in the top of the skull. What is its color? 
Note the parts of which it consists. Extending posteriorly from 
the brain is the spinal cord. How is it protected? Find some of 
the spinal nerves which emerge from the sides of the spinal column. 
Note that these are paired. The main part of the nervous system 
is in the dorsal region. Compare the nervous system with that of 
an invertebrate. 
EXERCISE 79 
THE METAMORPHOSIS OF A FROG 
Collect a number of frog eggs in March or early April. Place in 
jars or aquariums in which some green water plants are present. 
Observe from day to day. A series of drawings and of brief notes, 
dated as the observations are made, will make a satisfactory record 
of what takes place in the development of the egg and young frog. 
Note any changes in form, size, and activity. How long is it 
before the tadpole in the jelly-like envelope begins to move? 
When does it escape from this covering? Note the head. Note 
the position and size of the mouth. When do the external gills 
disappear? When do the internal gills first appear?" After the 
disappearance of the external gills, note the operculum. Through 
how many opercular openings does water escape after its circula¬ 
tion through the gills? On which side of the body is this opening? 
Compare the body of the tadpole with that of a fish with respect 
to its shape, gills, tail, and fins. What is the use of the tail? From 
the ventral side, try to see the coiled digestive tube. (This is 
adapted for the digestion of vegetable food.) Compare with that 
of the adult. How does the food of the adult differ from that of 
the tadpole? Which legs appear first? How is the disappearance 
of the tail related to the development of the legs? (The materials 
in the tissues of the tail are eventually absorbed and utilized during 
the development of the legs and of the remainder of the body.) 
