252 PROFESSOR MARSHALL AND EDWARD J. BLES. 



this communication is a very narrow one, and is seen more readily 

 in the third than in the second arch. It is shown on the right side 

 of Fig. 15 in the third arch (Fig. 15, A E). 



/. The Fourth Branchial Arch. — We are uncertain as to what is the 

 normal condition of the vessels in this arch at the stage we are 

 describing. In some specimens we find well-formed afferent and 

 efferent vessels as in the earlier stage, while in others, although a gill 

 is present, we have seen only a single vessel. We are inclined to 

 regard the vessels of this arch as in the act of transformation, the 

 afferent and efferent vessels communicating freely at the base of the 

 gill, and the upper or distal part of the afferent having already 

 disappeared in some specimens. 



From the dorsal part of the efferent vessel, just before it opens 

 into the aorta, the cutaneous and pulmonary arteries are given off as 

 before. * 



g. The Dorsal Aorta. — "We have nothing to say concerning the 

 aorta at this stage, except to note that the glomerulus is slightly 

 smaller and more pigmented than before. The lung, which lies 

 immediately ventral to the glomerulus on each side, is fused along 

 its outer wall for a short distance with the somatopleure opposite 

 the head kidney. The part of the body cavity in which the 

 glomerulus lies becomes thus partially shut off from the rest of 

 the coelom, with which, however, it remains in communication 

 posteriorly, and also, though by a more restricted passage, anteriorly, 

 in front of the root of the lung. 



5. General Considerations. 



There is nothing special to note at this stage except the formation 

 of the carotid gland, which has already been considered under the 

 previous stage. 



VIII. THE CONDITION OF THE HEART AND BLOOD VESSELS 

 IN TAILED FROGS DURING THE METAMORPHOSIS. 



1. External Characters. 



The stage we have selected for description is one in which the 

 total length is 22 mm., of which 14 mm. belong to the head and 

 trunk, and 8 mm. to the tail. 



