248 PROFESSOR MARSHALL AND EDWARD J. BLES. 



it is a specially acquired structure, formed by elaboration of the 

 direct passage between afferent and efferent branchial vessels which is 

 present in a simpler form on the hinder arches as well. 



There are two other points that deserve some attention. 



In the first place, Goette* figures and describes the third and 

 fourth branchial efferent vessels in Bombinator as not opening 

 directly into the dorsal aorta, but as uniting together at their dorsal 

 ends and running back as the pulmonary artery. Our descriptions 

 and figures, with those of Maurer, show that this is not the case in 

 Rana, the third and fourth branchial efferent vessels opening into 

 the aorta in the same manner as the first and second vessels. 



Lastly, our figures and descriptions show clearly that both the 

 cutaneous and pulmonary arteries arise from the fourth branchial 

 efferent vessel, a point in which we agree with Boast and differ from 

 Goette, who derives the cutaneous artery from the third arch. 



VII. THE CONDITION OF THE HEART AND BLOOD VESSELS 

 IN 21 mm. TO 23 mm. TADPOLES. 



1. External Characters. 



In tadpoles of from 21 mm. to 23 mm. length the general pro- 

 portions have not altered much. The head and body form a wide 

 rounded mass, about one-third of the entire length of the animal. 

 The lips bordering the mouth are very prominent and provided with 

 numerous teeth. The opercular spout is a cylindrical tube running 

 from the ventral surface backwards and upwards along the left side 

 of the body, and opening by a rounded aperture about the middle 

 of the length of the body. The suckers have entirely disappeared : 

 the nostrils are small, the eyes larger and more prominent than 

 before. 



The rectal spout opens at the ventral border of the fin by an 

 aperture inclined somewhat obliquely towards the right side. At 

 the base of the spout are the hind limbs, which, though larger than 

 before, are still mere papilla?, about half the length of the rectal 

 spout. 



* Goette, " Die Entwickelungsgeschichte der Unke," p. 753, and plate xxi., Fig. 377. 

 t Boas, "Ueber den Conus Arteriosus und die Aortenbogen der Amphibien," 

 " Morphologisches Jahrbucb," vii., 1881, p. 548. 



