i?4 FRANCIS VILLY. 



In tadpoles of about 8 mm., the full number of clefts is already 

 recognisable. They consist of five pairs of solid hypoblastic out- 

 growths, the four posterior pairs having met and blended with the 

 external epiblast, whilst the foremost or hyomandibular cleft ends in 

 the mesoblast some distance from the exterior (see Fig. 6). This 

 stage represents the highest point in the development of the hyoman- 

 dibular cleft, and after this period it commences to become smaller, 

 never acquiring a lumen for more than a short part of its length, and 

 never opening to the exterior. In tadpoles of a slightly larger size 

 (10 mm.) there is very little trace of the cleft in the form found in 

 the previous stage; but between the hyoid and mandibular arches in 

 the position where the cleft, at the stage just described, joined the 

 oesophagus, a small diverticulum of the gut is to be seen, and this no 

 doubt is the opened-out hyomandibular cleft. This pouch scarcely 

 has any existence of its own, appearing more as a sacculation of the 

 wall of the anterior part of the first branchial cleft than as an inde- 

 pendent gill cleft. 



It is at this time more ventral than dorsal in position. As the 

 tadpole grows it becomes less and less apparent, until it altogether 

 disappears, or at least can no longer be recognised as bearing any 

 resemblance to a cleft. The latest period at which it may be readily 

 recognised occurs in tadpoles of about 20 mm. It is not until the 

 hyomandibular cleft has ceased to be recognisable that the first trace 

 of the Eustachian tube appears. The Eustachian tube, therefore, 

 is probably not formed from the hyomandibular cleft, but is an 

 altogether new organ. Its development may be divided into two 

 periods, separated from each other by a stage, during which the 

 metamorphosis from the first to the last condition takes place. The 

 first beginning of the tube may be recognised in large tadpoles which 

 have attained approximately their full size, but before the hind legs 

 are conspicuous or toes are apparent. My youngest tadpole in 

 which this organ is present is 25 mm. in length. At this time the 

 Eustachian tube is very poorly developed, and consists of a solid 

 rod of cells running forwards from the dorsal and anterior edge of 

 the first branchial cleft under the palatopterygoid cartilage for a 

 short distance. This rod is very thin and not at all conspicuous ; 

 but it may be recognised by its position in relation to the first 

 branchial cleft. Unfortunately, I have cut no sections between this 



