I.] THE FROG. 165 



. The tail ; in all probability beginning to shrink. 



/. The young frog shortly after the moult (loss of the 

 larval skin) : characterized by the coexistence of 

 both pairs of limbs and the tail. 



Note its frog-like characters and especially 

 a. The mouth ; gape-wide, sucking lips gone, horny 



teeth replaced by true teeth. 

 /3. The tail; absolutely shorter than in the late 



tadpole. Note its relations to the trunk, 

 y. The doacal orifice. Although displaced, it still 

 lies at the base of the tail (i.e. it is ventral in 

 position. Cf. the adult). 



q. Look for specimens showing the mode of liberation 

 of the fore limbs. The left one almost invariably 

 appears first, forcing its way through the spiraculum ; 

 the right one is set free by absorption of the 

 opercular membrane. 



ii. The formation of the embryonic layers and certain 

 of the more important organs. 



Imbed and cut sections as directed below. Stain- 

 ing is unnecessary. 



a. At the first cleavage; longitudinal and at right 



angles to the cleavage furrow. 



u. The blastomeres ; equal in size and separated by 

 the cleavage furrow. Protoplasm ; note its pig- 

 ment and yolk granules. 



/?. Nuclei. These may or may not be visible ; 

 each is clear and transparent and generally 

 surrounded by pigment. (If visible, look for 

 indications of division.) 



