THE DEVELOPMENT OF THE FROG 



101 



tadpole. Subsequently these depressions meet in front and 

 become converted into a U-shaped groove. In the poste- 

 rior pair of plates, the gill plates, there appear two vertical 

 grooves, which later become converted into the gill slits ; 

 later two additional slits appear, one before and one behind 

 the other two, but none of them break through until after 



flG. 24. Embryos. Gp, gill plate ; Gs, Gs', two gill slits ; S, suckers ; Sp. 

 sense plate ; An, anus. (From Morgan, after Schultze.) 



the tadpole leaves the jelly. In the middle line, just 

 above the ventral sucker, the beginning of the mouth 

 appears as a hollow depression of the ectoderm, but it 

 does not communicate with the archenteron until a much 

 later period. The anus begins as an invagination of the 

 ectoderm a short distance behind the point where the 



