DEVELOPMENT OF EAR AND AOCESSORY ORGANS IN FROG. 541 
when the fore-legs are protruded. When this stage is reached 
the Eustachian tube has grown forwards, lying close to the 
palatopterygoid bar, and nowit extends to a point just in front 
of the eye. Though a lumen is not apparent the cells com- 
posing 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 13 
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 wall of the pharynx 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, keep- 
ing pace with the shifting of the hyoid 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 cesophagus 
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 terres- 
trial mode of life, accompanied by a rapid alteration of the man- 
dibular 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 
