INTRODUCTION 3 



Laboratory Manual) is preferable for this purpose. The 

 Anura or tailless amphibia are characterized not only by 

 the lack of a tail, but by the short broad trunk and the 

 fact that the hind legs are much larger than the fore legs. 

 The vertebrae are generally prococlous and the caudal 

 vertebrae are fused into a single bone, the urostyle. No 

 ribs are present. The skull shows certain peculiarities such 

 as the fusion of the frontals and parietals, and the presence 

 of a columella. A clavicle is present in the shoulder girdle, 

 and the ilium of the pelvic girdle is greatly elongated. The 

 radius and ulna are fused into a single bone, as are the 

 tibia and fibula. In the respiratory system, no distinct 

 trachea is found. The fifth aortic arch is absent in the 

 adult. The kidney is shorter than in the Caudata. 



The tissues of the frog are well adapted to serve as an 

 introduction to histology. The cells are large while the 

 organs are small enough so that sections through entire 

 organs can be viewed within the field of the low power 

 objective. Examples of all common vertebrate tissues may 

 be found. 



The embryology of the frog presents the following points 

 of interest. Fertilization is external. The egg is fairly 

 large and segmentation is quite regular through the first 

 cleavages. Hemisections through the blastula and gas- 

 trula are instructive at low magnifications. The young 

 hatch as larvae (tadpoles) with first external and then 

 internal gills. The large tadpole of the bullfrog is easily 

 dissected. Stages in metamorphosis may be demonstrated. 



Dissection is a technique which requires, above all things, 

 both care and patience. Follow directions carefully. Never 

 cut anything away without knowing what you are cutting 

 and what you are looking for. If you get in trouble, stop, 

 wash the specimen under the tap and make a fresh start. 

 Keep your dissecting instruments clean and sharp. Clear 



