OP ARTS AND SCIENCES, 117 



is seen to have a more ventral position (p. 7). The evidence that the 

 fissures are not closed by a direct fusion of the facing walls of the arches 

 Urbantschitsch finds in the fact that the width of the cleft does not suf- 

 fer any perceptible change in the different stages, whereas a reduction 

 of its length is directly visible at a comparatively early stage. " Fur- 

 thermore," he saj's, " one can easily find in successive cross sections, 

 either that the space between two arches discloses only the apposed 

 layers of the ectoderm, or that between these two layers there are pro- 

 ducts of the mesoderm. Where there is no union the section passes 

 through the region of the branchial fissure ; where the two apposed 

 layers of the ectoderm are encountered, there the cross section passes 

 through the most peripheral part of the crescentic outgrowth ; and 

 where the provertebral mass is found between the two layers of ecto- 

 derm, there we have the already completely closed branchial fissure." 



While the fissures are thus closed by the advancing mesodermic 

 encroachments, the oral sinus undergoes a differentiation into a central 

 and lateral portions ; the latter become enlarged, and communicate 

 with the former only by narrow fissures ; the lateral enlargement be- 

 comes the middle ear, and the fissure the Eustachian tube, so that in 

 its origin the middle ear is in no way derived from the first branchial 

 cleft, but is to be souixht in the two lateral sinuses of the oro-naso- 

 pharyngeal cavity. The tympanic membrane as seen from the outside 

 is at first not distinguishable, inasmuch as it lies in a plane with the 

 external surface of the rest of the embryo ; from the beginning it con- 

 sists of three layers, — the external layer of ectoderm, the mesoderm, 

 and the inner or oral layer or ectoderm (p. 13). 



Only occasionally is its position indicated externally by a slight de- 

 pression. This depression is the beginning of the external meatus, the 

 entrance to which is separated from the first visceral cleft by a narrow 

 bridge of tissue. Although this at first appears to lend support to the 

 theory of its derivation from the first cleft, it is nevertheless not so 

 derived. The orifice is the opening to a canal (meatus) which is 

 entirely distinct from the cleft, and forms with it an angle the opening 

 of which is directed inward and forward. This canal is formed by the 

 elevation of a ridge of tissue around this depression, rather than by any 

 ingrowth of the latter. 



Urbantschitsch ('78, p. 132) in a subsequent paper reaffirms his 

 conclusions, and claims that the external meatus is formed in the 

 embryos of mice in a like manner. 



By far the most careful and systematic description of the develop- 

 ment of the middle and external ear is that given by Moldenhauer ('77) 



