128 PROCEEDINGS OF THE AMERICAN ACADEMY 



remnant of the cleft into two distinct part?, a dorsal shallower and 

 a ventral deeper part. Both of these persist : the ventral part is trans- 

 formed into the deep portion of the meatus, and together with the supe- 

 lior half it also forms the cavity of the external ear. Therefore, as 

 claimed hy Reiehert, the ventral division of the cleft plays the more 

 important part in the formation of the external ear. In my observa- 

 tions I have been unable to lind any evidence in favor of Giinther's 

 view that the walls of the cleft fuse in such a maimer as to leave 

 for a short period two openings into the pharynx. On the contrary, 

 the bridge of tissue that closes the last remnant of the opening is a 

 simple accumulation of cells, which forms an uninterrupted membrane 

 of approximately uniform thickness. This remains for some time very 

 thin, and might easily be ruptured even by as delicate a probe as a hair; 

 moreover, the cleft does not show a defniitc division into a dorsal and 

 a ventral part until the embryo is about 10 mm. long ; but at this time 

 the cleft has long been closed, although the bridge is still very thin. It 

 is from these considerations that I have been led to believe that the 

 two openings claimed by Guuther were artificially produced by a rup- 

 ture of the tissue forming the bridge. 



As to the conclusions reached by Urbantschitsch about the manner 

 in which the visceral fissures close, it is to be observed, in the first 

 place, that his cross sections {op. cit., Figs. 1-3) do not afford the least 

 direct evidence that the first cleft remains pervious until it is reduced 

 to a mere hole, for the sections only pass through the orbital process 

 and the lower-jaw branch of the first arch. It is unfortunate for his 

 claim, that he has not reproduced the sections which show that " the 

 same condition that is encountered between the orbital process and the 

 lower-jaw branch is found between each two of the arches." In the 

 second place, the single section shown in his Fig 4 is not in itself alone 

 sufficient to prove that the cleft is not already closed by a continuous 

 brid«fe of tissue. He calls this a frontal section : the regions of the 

 embryo touched by the section seem, however, to indicate that it is not 

 strictly frontal, but, on the contrary, quite oblique to that plane, — 

 p(;rhaps even nearer a sagittal than a frottal plane. The section is to 

 all appearances approximately parallel to a tangent to the side of the 

 head and neck in the region of tlie visceral arches and clefts, so that it 

 cannot be safely inferred from tlie condition of the fissures in the plane 

 of this section that they present the same condition in every part of 

 their depth. The figures accompanying the present paper (Figs. 9, 10, 

 in show how unreliable the conclusions based upon any single section, 

 like that shown in Fig. 0. would have been. While, then, I do not 

 doubt tliat the restriction of the fissure is largely accomplished, as 



