NERVOUS SYSTEM (^^J 



to blastopore runs a slight groove, the primitive groove. Above 

 it a knob grows out to form the tadpole's tail. Grooves appear on 

 each gill plate marking out the visceral arches, and upon the first 

 two branchial arches branched external gills grow out. Below 

 the head, where the sense plates joined, a median pit of ectoderm 

 forms the stomodaeum, which will eventually break through to 

 the enteron and become the mouth. Below the stomodaeum is a 

 horseshoe-shaped sucker ; above, a pit in each of the sense 

 plates gives rise to the olfactory organ. When these changes 

 are complete the animal hatches. This happens about a fortnight 

 after the eggs are laid. 



EXTERNAL FEATURES OF LARVA 



In the external development of the tadpole the following 

 changes take place. A third pair of external gills is formed, and 



Sim 



Fig. 497. — A frog embryo at the stage of hatching. 



an., Proctodaeum ; au.c, slight swelling over the rudiment of the ear ; e.g., external gills on gill arches; 

 na., invagination to form nasal capsule ; o.c, slight swelling over the rudiment of the eye ; s., sucker; 

 stm., stomodaeum (invagination which will form the mouth). 



the mouth opens and is provided with a pair of horny jaws, 

 which are eventually lost. Meanwhile four gill-clefts open, and 

 the external gills wither, being replaced by new gills on the walls 

 of the clefts. The latter represent the first to fourth branchial 

 clefts of the dogfish, the external gills standing on the first three 

 branchial arches. Shortly after the appearance of the clefts a 

 fold of skin grows back from each side of the head so as to cover 

 them. The folds are the opercula ; they meet vontrally, and 

 presently their hinder edges fuse with the body everywhere, 

 except in one spot on the left side, where an opening is left for 

 the discharge of the water used in breathing. The sucker now 

 begins to degenerate. Shortly afterwards, rudiments of the hind- 

 limbs appear at the base of the tail, as a pair of small knobs, 

 which increase rapidly and become first jointed and then divided 



