DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 209 



mouth; and the longitudinal ridge, prolonged backwards from its 

 hinder border and marking the position of the lens of the eye, is 

 rather more prominent. The position of the ear can be recognised 

 as a slight swelling on the surface. The two external gills of each 

 side are larger ; that on the first branchial arch is notched at its 

 free posterior border into three blunt lobes, and that of the second 

 arch into either two or three lobes. The third branchial arch has 

 no external gill as yet. The Iryoid arch is more clearly marked out 

 than before, and slight vertical grooves indicate the positions of the 

 future gill clefts. 



2. Internal Anatomy. 



The internal structure differs but little from that of the 4| mm. 

 tadpole. There is, as might be expected, a certain amount of varia- 

 bility ; tadpoles of the same actual length not agreeing precisely in 

 their anatomy. In a certain number of specimens we have noticed 

 distinct asymmetry, the right side of the animal being slightly in 

 advance of the left in its development ; but whether this difference 

 is constant we have failed to determine. 



The fourth branchial arch is now more clearly defined than before ; 

 and the several gill pouches have begun to open out, the two layers 

 of hypoblast that form their walls separating slightly from each 

 other, so that the cavity of the pharynx now extends outwards as a 

 narrow cleft a short distance along each diverticulum. 



The lungs are slightly larger than before, but are still very small. 



3. The Heart and Pericardial Cavity. 



The pericardial cavity is much as before. Its floor and sides are 

 lined by a thin layer of flattened epithelial cells ; its posterior wall is 

 formed by the anterior surface of the liver diverticulum. Below and 

 at the sides of the liver the mesoblast is split into somatic and 

 splanchnic layers, and the ccelomic space between them, which is 

 still very narrow, opens into the pericardial cavity. 



The twisting of the heart (Plate XIV., Fig. 6) has increased very 

 considerably. From the sinus venosus the heart runs forwards to the 

 left side of the pericardial cavity, then bends sharply downwards, and 

 runs transversely across to the right side, and then bends sharply 

 forwards, upwards, and to the left, reaching the anterior wall of the 



