CHORDATA 



323 



Identify the axial skeleton and the appendicular . Do the appendages unite directly 

 with the axial skeleton? Count the vertebrae. To what extent do they differ? 

 Can they be grouped into regions? Select a typical one, and draw from various 

 positions to show structure. Do any bear ribs? Describe the posterior, bone in 

 the series. Identify the parts of the anterior and posterior limbs and girdles by 

 referring to Fig. 161, and see to what extent they depart from the type described 

 there. Make outline sketches of all the bones of the right girdles and appendages. 

 What is the nature and action of the various joints of the limbs? In the skull 

 notice how small a portion is brain-case. How is the great width of the head 

 secured? How is the lower jaw related to the skull? Make 'a sketch showing 

 the proportions of these various parts. In what position are teeth borne? Ex- 

 amine the sternum or breast bone. How related to the girdle? Of what parts is it 

 composed. How much of it is cartilaginous? 



8. Development. Eggs of frogs and toads may be found in the early spring in 

 ponds or sluggish streams, floating or attached to submerged objects. They 

 occur in slimy strings or masses, each egg enveloped in a jelly-like covering. 

 Transfer these to the laboratory, and keep covered with water in a shallow vessel. 

 Change the water frequently, and keep a close watch on the changes which they 

 undergo. After hatching keep a few water plants in the vessels for the tadpoles to 

 eat. 



Note appearance of the egg (with low power of microscope). 

 Gelatine: outer layer, not really a part of the egg. 

 Fertilized ovum; the darker interior sphere, of protoplasm and yolk. 



If the eggs are recently laid, the beginning of segmentation will furnish an 

 interesting demonstration for the class. How are the first cleavage planes related 

 to each other? 



If more advanced, note especially: the gradual elongation of the embiyo, the 

 enlargement of the head, development of the tail, hatching, the external gills. 

 What becomes of the gills? Do you find any trace of mouth, eyes, nasal openings? 

 Where do the legs first appear? What becomes of the tail? Prove. Tadpoles 

 of all ages may usually be found in the shallow ponds late in spring. These should 

 be compared with those reared in the laboratory. Dissect one of the larger tad- 

 poles, and examine particularly the intestine and the gill chamber. 



343. Compare with the frog any other amphibian types which can be found, 

 as the toad, the newt, or the salamander. Note especially the differences in habits, 

 haunts, external form, appendages, method and time of depositing eggs, the form 

 of the tadpoles, etc. 



DESCRIPTIVE TEXT 



344. General Characters. In common with the simpler 

 Chordata thus far considered the Vertebrata are bilaterally 

 symmetrical Metazoa with a ccelomic cavity, a notochord de- 

 rived from the entoderm, gill-slits at some stage of life, dorsal 

 nerve tube and a ventral heart. In addition, the following 

 points may be given as distinguishing the true vertebrates: 



i. The notochord comes to be surrounded by a sheath of 

 tissue derived from the mesoderm. This produces around the 



