DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 231 



are the perforation of the mouth, the opening out of the gill clefts, 

 and the solid condition of the oesophagus. 



VI. THE CONDITION OF THE HEART AND BLOOD VESSELS 

 LN 12 mm. TADPOLES. 



1. External Characters. 



The most marked difference in external appearance, as compared 

 with the 9 mm. tadpole, is the disappearance of the neck, owing to 

 growth back of the opercular fold, and fusion of its posterior edge 

 with the body wall. The head and body now together form an 

 ovoidal mass about one-third of the entire length of the animal, for 

 the tail has increased considerably in length. The tail is also much 

 wider than before, owing to increase in the height of the ventral 

 and dorsal fins, the latter of which now extends forwards along the 

 head as a low median ridge almost to its anterior end. 



The parts about the mouth- opening have grown forwards con- 

 siderably, so that the nostrils are situated further back on the sides 

 of the head. The eyes, though still small, are more prominent than 

 before. The mouth-opening is bordered by prominent lips, bearing 

 rows of small horny teeth, the lower lip being much the larger. 

 Within the lips lie the large horny jaws. 



The two halves of the sucker have now separated from each other, 

 and are placed some distance apart on the under surface of the 

 head. 



The opercular membrane is fused at its posterior edge with the 

 body wall, along the right side and across the ventral surface ; on 

 the left side of the body it is free, and prolonged backwards as a 

 short tubular spout, through which the tips of the external gills 

 commonly protrude. The opercular membrane is thin and semi- 

 transparent, and through it the gills and heart can readily be seen in 

 a living tadpole. 



The abdominal region is smaller than before, and somewhat pyri- 

 form in shape, tapering posteriorly to the long conical rectal spout, 

 which opens at the margin of the ventral fin. 



At the base of the rectal spout, in the angle between this and the 

 muscles of the tail, the posterior limbs are visible as a pair of small 

 rounded papillae, 



