OF ARTS AND SCIENCES. 121 



tympanic membrane arise exclusively from the substance of the man- 

 dibular arch.* 



Observations and Conclusions. 



Does the external passage of the ear arise by a transformation 

 of the first, or hyomandibular cleft? This question may be fully 

 answered by observing the external changes in a series of embryos. 

 It is to these external changes that I shall first direct attention. 

 There are three possible a j^riori answers to the question : first, the 

 external passage may be developed exclusively from the remnant of the 

 cleft, simply by an outgrowth of its walls ; secondly, it may arise quite 

 independently of the cleft, by means of a new invagination and subse- 

 quent changes ; or, finally, it may owe its origin in part to a transfor- 

 mation of the cleft, and in part to other and subsidiary changes of parts 

 not belonging to the walls of the cleft proper. 



The smallest embryo I have studied in this connection is G mm. 

 long ; t its form is much more simple than that of any of the later 

 stages. The visceral arches are straight, finger-like processes, without 

 elevations or depressions. The clefts are also straight and deep, with 

 perfectly smooth walls ; in the profile view of the embryo they extend 

 dorsally for nearly half the breadth of the neck (Fig. 1). In this 

 embryo only the mandibular and hyoid arches have become well de- 

 fined. The otic vesicle is still traceable by an external depression due 

 to a sinking in of its thin outer wall during the process of hardening. 



In the growth of an embryo from the 6 mm. to the 8 mm. stage 

 (Fig. 2), a considerable alteration takes place in the appearance of the 

 head and neck. The different regions of the brain have become well 

 marked externally ; the third visceral arch is sharply defined ; the 

 mandibular and hyoid arches are swollen at their roots. The furrow 

 described by Hunt as lying above the root of the mandible has become 

 perceptible (Fig. 2, d H). The mandibular and hyoid arches have 

 exchanged their finger-like appearance for somewhat club-shaped forms. 

 The a2:)pearance of the first cleft has also become altered ; formi^rly 



* Unless I have mj'self misunderstood the figures and meaning of Molden- 

 hauer, Kolliker is in error wlien he makes the latter responsible for tlie view 

 that the external meatus is derived from the most posterior (dorsal) part of the 

 cleft. I understand Moldenhauer to say that it is not the dorsal depression cor- 

 responding to the last closed part of the cleft, but a more ventral region of the 

 cleft, which marks the position of the future meatus and its floor, — the tym- 

 panic membrane. 



t Measurements were made in a straight line from the elevation of the mid- 

 brain to the most distant point of the embryo as it lay in its natural position. 



