OP ARTS AND SCIENCES. 129 



Urban tschitsch claims and as Reichert long ago stated, by the dorsal and 

 ventral ingi'owth of tissue, it remains none the less true, for the pig at 

 least, that this is only a part of the process, and that an early fusion 

 of the arches makes the first cleft impassable long before the reduction 

 in the length of the fissure is effected by these mesodermic growths. 

 Moreover, although I admit the dorsal ingrowth to be subsidiary to an 

 increase in the thickness of the partition that closes the cleft, it helps 

 only after the bridge has been formed by cells derived from the hyoid 

 arch. 



The conditions presented in the chick, as shown by Moldenhauer's 

 studies, and those in the pig, are in many points strikingly similar ; 

 but it is the interpretation given to the facts by Moldeuhauer which is 

 at variance with my own conclusions. The real question at issue is 

 this : Is any part of the tubo-tympanic sulcus a result of the direct 

 metamorphosis of the first cleft, or is it rather the result of a secondary 

 outgrowth from pharyngeal cavity ? Moldenhauer defends the latter 

 assumption, whereas I am convinced of the validity of the former. To 

 answer the question, it seems to me only necessary to determine the ex- 

 tent of tlie tubo-tympanic sulcus in two directions, — the dorsal and the 

 posterior.* According to his own figure (Moldenhauer, '77, Taf. VIII, 

 Fit^ 23), with which m}' own on the pig (Fig. 16) agrees in all essen- 

 tial matters, the dorsal limitation (blind end) of tlie sulcus corresponds 

 quite accurately with the dorsal extent of the fissure before its closure. 

 The depression of tlie ectoderm dorsal to his colliculus posterior, 

 throu"'h which the lints a h pass in his Fig. 23, lie, accnrding to his 

 own description (p. 130), in the prolongation of this sulcus; but the 

 depression in question corresponds approximately to the dorsal limit of 

 the now closed cleft. That this depression does not subsequently grow 

 deeper and thus become the external meatus, as has been supposed by 

 some of the earlier observers, but is finally obliterated, can be no argu- 

 ment apaiust its being the remnant of the dorsal region of the primitive 

 cleft ; if it is such, then it is certain that the sulcus tubo-tympanicus does 

 not have a greater dorsal extension than v/ould be expected if it were 

 simply the remnant of the inner half of tlie first cleft. There is, then, 

 no motive for assuming a secondary outgrowth in a dorsal direction. 



As regards the anterior (upward, IMoklinhauer) extent, it certainly 

 must be admitted to be at first greater than that of the fissure ; its pos- 

 terior end, however, communicates at an early stage with the outside by 



* Tostcrior and superior respectively, according to Moldenhauer's tcrmi- 

 nolosy. 



VOL. XIX. (N. S. XI.) 9 



