lit) PROCEEDINGS OF THE AMERICAN ACADEMY 



the boundary of the swollen root of the first branchial arch and the 

 neigliboriiig tissue. " The Eustachian tube," he says ('77, p. 9), " is 

 an involution of the mucous membrane lining the pharynx; it is not in 

 any way the remains of a branchial fissure, but lies in the tissue in 

 wliich the base of the skull is forming; it grows in length as the bran- 

 chial fissure closes." lie states that in cross sections of the head, which 

 incline anteriorly as they pass downward so as to cut the first arch 

 obliquely, the Eustachian tube can be seen as a slight depression in the 

 roof of the pharynx. In an embryo nine sixteenths of an inch long it 

 becomes deeper, pointing upwards and outwards. In an embryo eleven 

 sixteenths of an inch in length the Eustachian tube overlaps the inner 

 end of the meatus. The tissue separating the meatus and Eustachian 

 tube becomes transformed into the tympanic membrane. 



About the same time the development of the accessory parts of the 

 ear was made the object of special studies by Moldenhauer and Ur- 

 bantschitsch. Moldenhauer ('76) published a preliminary account of 

 his results in the latter part of 1876; his conclusions will be given a 

 little further on. 



The studies of Urbantschitsch ('77) were made upon the rabbit and 

 the chick. Supporting his statements (p. 4) by well-known conditions 

 found in longitudinal sections of batrachians, he claims that the ridge- 

 like thickenings of epithelium which mark the boundary between the 

 fore-gut and the oral invagination (i. e. between entoderm and ectoderm) 

 correspond exactly in position with the subsequently developed larynx. 

 The oro-naso-palatal sinus in birds and mammals, which is at first much 

 less pronounced than that of the lower vertebrates, is only definitely 

 formed when the front part of the brain has come to form an angle 

 with the axis of the rest of the body. It is then that the branchial 

 arches begin to form by their downgrowth the lateral boundaries of this 

 sinus, which is converted into the definite oro-naso-ixilatal cavity by 

 the ventral union of the arches. The branchial fissures, as well as the 

 inner wall of the arches and all other parts of this cavity, are therefore 

 lined with ectoderm. The usual statement that the fissures are closed 

 by the concrescence of the separate arches is not accurate ; the closure 

 of the clefts is effected by a crescent-shaped outgrowth of the substance 

 of the provertebra which lies opposite the dorsal end of the cleft (p. 6). 

 This outgrowth covers, or fills up, a portion of the dorsal part of the 

 cleft, and a similar growth from the ventral side diminishes its extent 

 in the opposite direction. Thus the fissure is reduced to a small cir- 

 cumscribed opening, — a temporary communication between tlio oral 

 cavity and the outside, — which, just before its complete disappearance, 



