120 PROCEEDINGS OP THE AMERICAN ACADEMY 



In corresponding sections through embryos of the ninth day the pos- 

 terior colliculus, as well as the depression behind it, no longer exists ; 

 the depressed region grows gradually deeper from behind (dorsal) for- 

 ward, where it is terminated by a steep wall, — the anterior colliculus. 

 An enlargement of the blind end of the sulcus opposite this depression 

 is the incipient tympanum. 



In an embryo of t'le twelfth day the external depression — the 

 meatus externus — has become deeper; the deeper portion of its pos- 

 terior wall is diflE'erentiated into a floor, the tympanic membrane, with 

 which the anterior wall makes a sharp angle. The tympanic mem- 

 brane is only iialf as thick as in the nine-day stage. Since it is not the 

 dorsal depression, lying in line of the prolongation of the sulcus, which 

 becomes the external meatus, but rather the more ventral depression, 

 Moldenhauer argues that the tympanic membrane has no relation 

 whatever to the blind end of the sulcus, its mucous surface corre- 

 sponding to a definite territory of the ventral region of the wall of the 

 front intestine. Similar conditions are maintained for the embryo 

 of the deer. 



From the study of frontal sections the author concludes that the 

 tympanic cavity lies on the inner side of the first arch, since in the chick 

 of the seventh day the lower (posterior) wall of the primitive tympa- 

 num lies nearly in continuation with the layer of epithelium which 

 indicates the position of the line along which the arches fused. The 

 tympanic membrane is formed at the expense of the substance of the 

 mandibular arch, not that of the hyoid. This conclusion is based prin- 

 cipally on the fact that the lower (posterior) margin of the first arch 

 gradually becomes thinner in the tympanic region, and that the upper 

 edge of the hyoid arch rises abruptly from the region of minimum 

 thickness. 



Kolliker (79, pp. 746-755) still holds that the tympanum and tuba 

 are undoubtedly developed from the median part of the posterior (dor- 

 sal) section of the first branchial fissure, which, however, is not directly 

 and without further change metamorphosed into these parts, but which 

 grows out into a hollow process directed outward, upwards (forward), 

 and backwards (dorsal). At the same time the external meatus, which 

 at first is shallow, forms also a hollow process which takes exactly the 

 opposite direction; the meatus therefore does not become deeper sim- 

 ply from the upgrowth of the tissue surrounding its external opening. 

 Kolliker finds in the rabbit nothin": to favor Moldenhauer's view that 

 the mouth of the tuba and that of the first cleft do not correspond, 

 neither is he willing to commit himself to the opinion which makes the 



