DEVELOPMENT OF THE EAR IN THE COMMON FROG. 175 



and a considerably later stage ; but notwithstanding this, I have 

 no hesitation in identifying the structure just described with the 

 Eustachian tube as it appears in tadpoles just before the time when 

 the fore legs are protruded. When this stage is reached the Eustachian 

 tube has grown forwards, lying close to the palatopterygoid bar, and 

 now it extends to a point just in front of the eye. Though a lumen 

 is not apparent, the cells composing the organ are arranged in the 

 form of a tube with walls more or less definitely one cell in thickness, 

 and at its distal end the tube is somewhat expanded. Figs. 11, 12, 

 and 1 3 will give an idea of the relations of the structure at this time. 

 At the commencement of the tadpole's metamorphosis the tube 

 loses its connection with the Avail of the pharnyx, and quickly divides 

 into a variable number of short lengths. This state continues until 

 a short time after the frog has lost its tail, and while it lasts the 

 broken fragments move backwards, keeping pace with the shifting 

 of the hyoicl arch. When the position of the Eustachian tube that 

 is to be has been reached by its components, they commence to grow 

 towards each other, and at the same time a hollow process of the 

 wall of the pharynx arises and completes the whole length ; a cavity 

 is acquired by the walls separating, and the adult form is attained 

 in all important points. This process is shown in Figs. 15 and 16. 

 The irregularity of this change through which the tube passes is very 

 marked. The number of pieces into which it divides differs much in 

 different specimens, some showing only two or three detached lengths, 

 whilst others possess a chain of such lengths consisting of several more 

 links. The two sides of the same specimen often differ considerably. 

 In one specimen the Eustachian tube is complete on one side and has 

 the adult form, whilst the other has only detached fragments (see 

 Fig. 16). No doubt this mode of change from the immature to the 

 adult condition is secondary, and abbreviated from the more natural 

 course of development ; the condensation of history being occasioned 

 by the need of a rapid change from aquatic to terrestrial mode of life, 

 accompanied by a rapid alteration of the mandibular and hyoid bars 

 in their position with regard to the rest of the skull. The slightly 

 expanded distal portion of the Eustachian tube described in its 

 second stage is retained to form the future tympanic cavity in 

 all specimens examined, and keeps near the surface, maintaining 

 relations with the future annular cartilage. 



