THE DEVELOPMENT OF VERTEBRATA, 



239 



most important structures which are developed before the 

 liberation of the embryo from the egg as a tadpole. This 

 takes place about a fortnight after fertilization, a long 

 period in comparison with the eight hours of Amphioxus. 

 One other temporary organ now developed must be men- 

 tioned ; this is a pair of suckers on the ventral surface, a 

 little behind the mouth (fig. 122 [1]). By means of these 

 the tadpole attaches itself to water- weeds (fig. 122 [2]) for 



Fig. 122. DE\ELOPMENT OF FROO. 



(1) Shortly before hatching. (2) Newly-hatched tadpoles. (3) Tadpole with ex- 

 ternal gills. (4) Tadpole with internal gills and operculum. (5) During the 

 metamorphosis. (0) Young frog. N.B. The scale of the six figures differs greatly. 



a few days, until the stomodseum is perforated and horny 

 jaws developed, when for the first time it begins to feed. 



The tadpole just hatched is at a far higher level of 

 development than the larva of Amphioxus when it begins 

 its free existence. The possibility of its attaining this level 

 while still within the vitelline membrane is due to the 

 relatively great amount of yolk stored up in the ovum. 

 Even at the time of hatching this is not all absorbed, though 

 it practically all is by the time active feeding has begun. 



