EMBRYOLOGY 55 



339. The larva at hatching ^ (6 mm.). Note the changes 

 that have taken place since the 3 mm. stage in the 

 structures mentioned above. On the third and fourth 

 visceral (gill) arches, external gills have appeared. 

 Dorsal and lateral to the margins of the stom- 

 odaeum are two minute olfactory pits which are 

 the beginnings of the nose. 



Drawing 32. Draw these three stages one above the 

 other, from the left side, magnified about twenty times. 



3. The Dissection of the Tadpole {Rana catesbiana) 



The bullfrog spends the first two years of its existence in 

 the larval or tadpole stage. The large size of the tadpole 

 offers an excellent opportunity for a dissection of the larval 

 organs and a comparison with those of the adult. 

 Place the tadpole on its right side and observe: 



340. the external characters. What changes have taken 

 place since hatching? Find the ventral mouth, 

 armed with horny teeth; the tiny nostrils, one about 

 a quarter of an inch dorsal of each side of the 

 mouth; the eyes; the opercular aperture half way 

 down the trunk on the left side; the tail, with its 

 fin; and at the base of the latter locate the tiny 

 hind limbs. The vent is at the base of the fin on the 

 right side. 



Drawing 33. Make a drawing of the tadpole from 

 the left side. Place the tadpole on its back and remove 

 the body wall from the ventral surface, taking care not 

 to injure the opercular aperture. 



* The larva of R. pipiens under laboratory conditions varies in 

 length from 3 to 6 mm. at the time of hatching. 



