54 EXPLANATION OP PLATES. 



Fin. 23. Section through the superior semicircular canal in a human fetus 130 mm. crown-rump length (Carnegie 

 Collection, No. 1018, slide 29, section 2), enlarged 90 diameters. The periotic reticulum is undergoing the 

 alterations characteristic of the early stages of the formation of tissue-spaces. Along the margins of the 

 cartilage the retieular tissue is condensed and constitutes the fibrous perichondrium. Around the epithelial 

 canal there is developed a layer of supporting tissue which forms the membrana propria. This layer fuses 

 with the perichondrium along the peripheral margin of the canal and thereby constitutes a ligament that 

 attaches each membranous duct throughout its whole length to the cartilaginous space in which it is sus- 

 pended. 



Flo. 2">. Section through the apex of the cochlea of a human fetus 130 mm. crown-rump length (Carnegie Collection, 

 No. 1018, slide 32, section 2), enlarged 57 diameters. This section shows the tip of the cochlear duct and 

 the character of the communication that develops between the two scalse forming the helicotrema. It 

 will be seen that the margins of the periotic spaces are not so mature here as in the proximal parts of the 

 cochlea of the same fetus, on comparing this figure with figure 20. 



PLATE 4. 



The figures shown on this plate represent a series of median and lateral views of wax-plate reconstructions of the 

 membranous labyrinth and the surrounding periotic tissue-spaces. They illustrate under the same scale of 

 enlargement three typical stages in the development of these spaces. Abbreviations: C. s. 1., ductus 

 semicircuiaris lateralis; C. s. p., ductus semicircularis posterior; C. s. s., ductus semicircularis superior; 

 Duct, coch., ductus cochlearis; Impressio rotund., area opposite the fenestra cochleae; Impressio staped., 

 area in contact with base of stapes; Saccus endol., saccua endolymphaticus; Scala tymp., scala tympani; 

 Sc.ala vestib., scala vestibuli. 



Fie 21). Lateral view of a model reconstructed from a human fetus 50 mm. crown-rump length (Carnegie Collection, 

 No. 84). The cistern and the scala vestibuli are shown in green and the scala tympani is shown in orange. 

 The scala vestibuli is in the first stage of its development and consists of a row of large retieular spaces which 

 extend from the ventral margin of the cistern downward along the apical surface of the cochlear duct. The 

 scala tympani is more advanced and shows more complete coalescence of its constituent spaces. Enlarged 

 11.4 diameters. 



FIG. 27. Median view of the same model shown in figure 26. This view shows the topography of the scala tympani. 

 Its large proximal end lies opposite the fenestra cochlea; (rotunda) and corresponds to the focus at which its 

 development originates. Distally it tapers off rapidly where the spaces are smaller and their coalescence less 

 complete. Enlarged 11.4 diameters. 



Fl(i. 28. Lateral view of wax-plate reconstruction of the left membranous labyrinth and the periotic spaces in a human 

 fetus 85 mm. crown-rump length (Carnegie Collection, No. 1400-30), enlarged 11.4 diameters. The cistern 

 and the connecting scala vestibuli are shown in green. Although the greater part of the cistern abuts against 

 the -stapes, it will be noted that it is also beginning to spread over the dorsal surface of the utricle and along 

 the inner border of the lateral semicircular duct. The scala vestibuli communicates freely with the cistern 

 and extends downward along the apical surface of the cochlear duct throughout nearly two turns, showing 

 the characteristic sacculated appearance near its tip, where the coalescence of the spaces is less complete. 



Fifi. 29. Median view of same model shown in figure 28, enlarged 11.4 diameters. The scala tympani is shown in 

 orange. The oval indentation in its proximal end corresponds to the fenestra cochleae (rotunda). This 

 space extends along the cochlear duct about the same distance as the scala vestibuli, but the two do not com- 

 municate yet at any place. The peripheral border of the scala tympani is characterized by sacculations 

 corresponding to spaces that are coalescing with the main space. The growth of the scala is due to a coales- 

 cence of new spaces along its peripheral border rather than along its central border. 



Fin. 30. Lateral view of a wax-plate reconstruction of the left membranous labyrinth and the periotic spaces in a 

 human fetus 130 mm. crown-rump length (Carnegie Collection, No. 1018), enlarged 11.4 diameters. The 

 cistern and scala vestibuli are shown in green and the scala tympani is shown in orange, as in the previous 

 figures. The cartilaginous stapes was removed from this model and the oval impression that it makes on the 

 cistern can be plainly seen. The cistern has spread over the top of the utricle and part way along the lateral 

 semicircular duct. The scala vestibuli extends to the tip of the cochlear duct, where it communicates with 

 the seala tympani, thus forming the helicotrema. 



FK;. 31. Median view of same model shown in figure 30, enlarged 11. 4 diameters. The oval impression on the proxi- 

 mal end of the scala tympani corresponds to the fenestra cochlere (rotunda). As yet there is no communica- 

 tion at this point between the scala tympani and subarachnoid spaces, such as is found in the adult and 

 known as the aquanluctus cochleie. The spaces making up the cistern cover almost, the whole of the utricle 

 and saccule except the places at which the nerves enter and a small part of the medial surface near the attach- 

 ment of the appendage. 



