224 PROFESSOR MARSHALL AND EDWARD J. BLES. 



contains two vessels, afferent and efferent, which lie close alongside 

 each other, and communicate directly at the tip of the lobe (Fig. 12), 

 the afferent vessel being placed posteriorly and somewhat ventrally 

 to the efferent vessel. The third branchial arch bears an external 

 gill which is much smaller than those of the first and second arches, 

 and almost concealed by these. The heart's beats are rapid — about 

 fifty to the minute. The whole surface of the tadpole is, as in the 

 earlier stages, ciliated ; the cilia working from head to tail, causing 

 the animal when perfectly quiet to move forwards slowly in the 

 water. Distinct opercular folds (Fig. 12, OF) are present on the 

 ventral surface of the head, growing backwards from the posterior 

 borders of the hyoid arches. 



2. Internal Anatomy. 



The proportions and relations of the organs in the anterior part 

 of the head have changed very considerably as compared with the 

 earlier stages, as will be seen on comparing Figs. 7 and 11. The 

 brain has grown forward very markedly, the cerebral hemispheres 

 being now conspicuous structures. The anterior lip has grown still 

 more rapidly, so that the brain, in spite of its forward extension, 

 no longer reaches to the anterior end of the body. This growth 

 of the lips causes considerable deepening of the stomatodseal 

 depression. 



From the bottom of each olfactory pit a solid extension of the 

 nasal epithelium has grown downwards towards the mouth, which it 

 nearly meets just opposite to the septum. A slight diverticulum 

 of the stomatodaeum immediately in front of the septum extends 

 towards it, but does not yet meet it ; these two structures together 

 forming the rudiment of the posterior narial passage. 



The pharynx is very wide from side to side, widest opposite the 

 second branchial arches; but is shallower dorso-ventrally than before. 

 The second and third branchial clefts (Fig. 12, C B.2, C B.3) now 

 open to the exterior, the opening being effected by separation of the 

 two hypoblastic lamellae of which each gill pouch consists, completed 

 by a very slight groove-like depression of the surface epiblast. The 

 openings are narrow slits lying immediately behind the first and 

 second external gills respectively, and rather on the ventral surface. 

 The opening of these two clefts to the exterior we have found to 



