66 OUTLINES OF CHORDATE DEVELOPMENT 



and in the head region are visible the rudiments of the man- 

 dibular and hyoid arches and the optic vesicle. 



The influence of the yolk mass upon the form of the embryo 

 now diminishes rapidly, and during the next few days external 

 change consists largely in the appearance of a definite body 

 region, the elongation of the tail, and the enlargement of the 

 head upon which appear olfactory pits, stomodaeum, and 

 sucking disc, and just back of the head the rudiments of 

 external gills and the pronephric elevations (Fig. 22, G). 



At about six days (5 mm.) the embryo begins to show muscu- 

 lar twitchings, and about one or two weeks after fertilization 

 the embryo wriggles its way out of the jelly and becomes a 

 free living larva or tadpole (Fig. 22, G). This marks the end 

 of the embryonic period. (In the higher temperatures of the 

 laboratory the larvae may hatch within five days after fer- 

 tilization.) For some days after hatching the larvae remain 

 comparatively inactive, sometimes attaching themselves by 

 their U-shaped suckers to the outside of the jelly mass, or to 

 other objects in the water and hanging thus, singly or in groups. 

 Or they may fall to the bottom and lie passively on one side. 

 During the days just after hatching the Iarva3 are still depend- 

 ent for food upon the yolk contained within the wall of the 

 intestine, but about two to five days after hatching the mouth 

 opening is formed, and the tadpoles begin to take in food from 

 outside. As the tadpoles begin to feed they become active, the 

 sucker becoming functionless and diminishing; and soon they 

 are in almost constant motion searching for food over the bot- 

 tom, or in the surface film of the water. The mouth becomes 

 fringed with lips, covered with horny rasping papillae and 

 furnished with a pair of horny beaks. Their food consists of 

 almost any kind of plant or animal debris and this is con- 

 sumed in immense quantities. In captivity tadpoles thrive 

 perfectly on a diet of any cereal, with the occasional sacrifice of 

 one of their own number. As the alimentary tract becomes 

 functional the digestive glands increase in size rapidly, and the 

 long intestine, coiled like a watch spring, can easily be seen 

 through the ventral body wall. This enlargement of the diges- 



