CLASS AMPHIBIA 537 



groove known as the neural groove. The neural folds or lateral edges 

 of the plate grow rapidly and curve in to meet in the dorsal midline, 

 where they fuse to form the neural tube. Anteriorly the neural tube 

 soon closes to end blindly while posteriorly it remains open to the ex- 

 terior at the blastopore for a time. Subsequently this communica- 

 tion with the blastopore closes. The neural tube soon separates from 

 the ectoderm above, its walls thicken and through other changes it 

 develops into the central nervous system. The anterior part becomes 

 the brain and the posterior part the spinal cord. 



During these changes, the egg, which previously was spherical, has 

 elongated in the axis of the neural tube. The blastopore has been 

 covered by the folding of edges of the neural groove extending to 

 its borders. Subsequently the embryo takes on form so that defi- 

 nite body regions can be identified. 



Later Development. — In the head region appears an elevated side 

 plate, the gill plate, where later the gills develop. Anterior to this, 

 a swelling on each side of the head denotes the beginning of certain 

 sense organs. A depression on the anterior ventral surface is a fore- 

 runner of the mouth, and posterior to this a crescent-shaped area 

 indicates the beginning of a ventral sticker by which the newly 

 hatched larva may attach itself to objects. At the posterior end of 

 the body a tail hud appears, and the region of the blastopore becomes 

 the anus. The embryo soon hatches, branched external gills which 

 serve as respiratory organs make their appearance, and definite sense 

 organs can be found on the head. This is the external gill stage. 

 The intestine becomes coiled ; the tail elongates ; and muscle segments, 

 or myotomes, can be seen along the sides of the body. Shortly after 

 hatching, the external gills are absorbed and internal gills take their 

 place. A fold of skin, the operculum, develops over this region. 

 There remains on the left side, however, a small opening, the spiracle, 

 through which water, taken in at the mouth, may pass out after going 

 over the internal gills. The skeleton is cartilaginous; lateral line 

 sense organs are present on the sides; the pineal organ is evident; and 

 the animal is similar to a fish. 



The tadpoles feed primarily on plant substances. In the mouth of 

 the bullfrog tadpole is one row of teeth above and three rows below, 

 plus a border of projections, known as papillae, for testing food sub- 

 stances. Between the lips is a horny beak somewhat like that of a 



