42 THE HISTOGENESIS AND GROWTH OF THE OTIC CAPSULE AND ITS 



In preparing this and the models shown in figures 28 to 31, it is to be remembered 

 that only those periotic spaces are included that were outlined by a membrane- 

 like margin. In the adjacent reticulum there are spaces that are actively coa- 

 lescing and gradually uniting with the main cavity. No attempt, however, was 

 made to show such spaces in the models. From figures 26 and 27 it will be seen 

 that the scala tympani is larger and more advanced in its development than the 

 scala vestibuli. The latter is in its earliest stage and consists of hardly more than 

 a row of enlarged reticular spaces which extend downward from the cistern along 

 the dorsal and apical surface of the cochlear duct. A section through the scala 

 vestibuli in another fetus of about the same age (Carnegie Collection, No. 448) is 

 roughly shown in figure 21, the spaces of the scala being situated between the 

 cistern and the cochlear duct. 



The scala tympani consists of an elongated oval space lying along the basal 

 surface of the proximal part of the cochlear duct, about corresponding to the prox- 

 imal half of the first turn of the duct. In the main part it is a single space with a 

 distinct margin separating it from the general periotic reticulum. In the more apical 

 portion it tapers off into multiple incompletely united smaller spaces which actively 

 coalesce as the process advances into the new territory along the duct. It is of 

 interest to note that the most mature and the largest part of this scala, representing 

 the focus at which it first appeared, is opposite the fenestra cochleae (rotunda), 

 just as the cistern forms opposite the stapes and the fenestra vestibuli. The scala 

 tympani always begins at the same place and extends downward along the cochlear 

 duct, at first a little in advance of the scala vestibuli, but subsequently the latter 

 catches up with it and the two reach the tip of the duct at about the same time. 



It is well known that the proximal portions of the cochlear duct mature sooner 

 than the distal portions. One might expect that the accompanying periotic spaces 

 would correspond in their development to the maturity of the duct and therefore 

 the proximal parts of the scalse would differentiate first. In other words, the 

 maturation of the cochlea proceeds as a wave from the proximal end to its tip, 

 involving all of its constituent structures as it passes along, including mesenchymal 

 parts as well as epithelial. 



This conception might explain the direction of development of the scala?, but can 

 hardly be applied to the cistern, the vestibular representative of the scala vestibuli. 

 One can not say that those portions of the membranous labyrinth lying oppo- 

 site the focus of development of the cistern (that is, the lateral walls of the saccule 

 and utricle) mature in advance of the rest of the labyrinth. There is no indication 

 that a wave of differentiation passes through the epithelial elements of the laby- 

 rinth in the same direction and synchronously with the extension of the cistern 

 as it advances from its primary focus upon the roof of the utricle and over on its 

 median surface. In the case of the cistern it seems much more likely that the point 

 at which it first appears is determined by the position of the stapes, which is doubt- 

 less an expression of the physical relation that subsequently exists between the two. 

 By analogy this would yield additional significance to the relation existing between 

 the fenestra cochlea? and the point of beginning development of the scala tympani. 



