46 THE HISTOGENESIS AND GROWTH OF THE OTIC CAPSULE AND ITS 



part of an adventitial coat to the completed scala. The trabeculse that separate 

 the enlarged spaces seem to be under tension and about ready to snap apart. In 

 fact, in most sections one can see the fragmentary ends of trabeculse where this 

 interruption of continuity has apparently occurred. 



The differentiation of the margin of the scalae constitutes the final feature in 

 their maturation. During the period in which the enlargement of an individual 

 scala is being brought about by the coalescence of enlarging reticular spaces, the 

 margins of the main cavity can be seen to consist of smooth, delicate strands of 

 nucleated protoplasm that resembles in all essentials that of the trabeculae between 

 the large reticular spaces. These linear margins are interrupted here and there 

 by openings into adjacent spaces, but they tend to form a continuous line that 

 definitely marks off the space from the adjacent reticulum. An early stage in the 

 formation of such a margin is shown in figure 25, where the margin is indicated at 

 a few places, but for the most part the space abuts against the surrounding ragged 

 reticulum. The margin of the space is more complete in the scala tympani shown 

 in figure 22, but it is still thin and delicate and can be easily opened up to allow the 

 taking in of new spaces. If we examine the borders of more mature spaces we find 

 them inclosed by a firmer membrane, which finally reaches a state that will probably 

 not admit of any further opening up for the coalesence of additional spaces. Any 

 further growth must thereafter be limited to simple distention of the wall of the 

 space with the consequent adjustment of its constituent cells. Such a condition 

 is represented in figure 24. This shows a more mature section of the wall of the 

 scala vestibuli, being a detail of the same section shown in figure 20. The only 

 difference between such a membrane, as we must now call it, and the corresponding 

 .structure in younger stages is its density; it is wider and its protoplasm perhaps 

 more opaque, or in other words, more protoplasm is accumulated there. 



If figures 24, 22, 25, and 23 are compared and followed in that order, it 

 will be seen that the lining membrane of the scalse can be traced backward, step 

 by step, to the ordinary trabeculse of the periotic reticulum. There is no histo- 

 logical evidence that any new cells enter into its formation. It seems to be simply 

 a product of the proliferation and adaptive reshaping of the cells already there. 

 In its final form the margin of the space resembles an endothelial membrane. One 

 could describe, as immediately lining the space, a thin membrane with flattened 

 nuclei, which is supported underneath by a thin coat of nucleated protoplasm that 

 has the form of fibrous connective tissue. The former, judging only from its final 

 appearance, one might designate as endothelium and thus make a distinction be- 

 tween it and the underlying tissue. In its histogenesis, however, it differs in no way 

 from the rest of the wall and the difference that exists later seems to be merely the 

 result of its adaptation to the existing physical conditions. Its early behavior is 

 entirely different from that of vascular endothelium. Thus if its final appearance 

 is stressed and the term endothelium is used for its designation, it must be done 

 with a considerable amount of reservation. It is preeminently a place where the 

 term mesothelium could be used with great advantage. 



