EXPLANATION OF PLATES. 

 PLATE 1. 



The figures on Plates I and II represent a series of photographs of the ear region in human embryos varying from 4 mm. 

 to 130 mm. long. The photographs were taken at a magnification of 100 diameters and as far as possible 

 at similar positions, so that a comparison of them would indicate the actual increase in size and the relative 

 amount and form of the individual tissue-masses. In the reproduction they were reduced to about 00 

 diameters. The different figures include the principal stages in the development of the cartilaginous 

 capsule of the ear and show the gross features of the histogenesis of the periotic reticulum. Figures 5 

 to 7 cover the period during which the mesenchyme becomes condensed around the otic vesicle. Figures 

 8 to 10 show the otic capsule in its precartilage stage and the manner in which the precartilage becomes 

 differentiated into relatively permanent and temporary zones. The latter encircle the epithelial ducts and 

 correspond to the future cartilaginous canals. In figures 11 to 13 the main capsular mass has become true 

 cartilage, whereas the temporary zone of preeartilage surrounding the canal is on the point of dedifferen- 

 tiating into periotir reticulum. A focal area of vascularized reticulum is already established at the inner 

 margin of the epithelial duct. 



FIG. 5. Frontal section through the region of the ear in a human embryo 4 mm. long (Carnegie Collection, No. 588, 

 slide 6, row 6, section 6). The section is 15ju thick and is enlarged 00 diameters. It shows part of the 

 brain-wall and the otic vesicle with the surrounding mesenchyme. The nuclei of the latter are more numerous 

 in the neighborhood of the vesicle, indicating the beginning of the capsular condensation. 



Fii. <i. Horizontal section through the region of the ear in a human embryo 9 mm. long (Carnegie Collection, No. 721, 

 slide 5, row 2, section 1). The section is 15ju thick and is enlarged 00 diameters. It shows a distinct con- 

 densation of the mesenchyme around the otic vesicle, particularly on its lateral surface (above) where it 

 extends from the surface of the vesicle to about half the distance from the vesicle to the ectoderm. 



FIG. 7. Frontal section through the labyrinth in a human embryo 11 mm. long (Carnegie Collection, No. 3.53, slide 16, 

 row 3, section 4). The section is lOju thick and is enlarged 90 diameters. It shows the vestibular part of 

 the labyrinth with the appendage opening out of it and passes transversely through the pouches whose 

 margins are to form the superior and lateral semicircular ducts. There is now a very complete capsule of 

 condensed mesenchyme surrounding every part of the labyrinth, with the exception of the appendage and the 

 regions of the internal auditory meatus and the fenestra cochlea?. 



FIG. 8. Horizontal section through the otic capsule in a human embryo 15 mm. long (Carnegie Collection, No. 719, 

 slide 3, row 2, section 3). The section is 40^ thick and is enlarged 90 diameters. It shows a portion of the 

 utricle below and the superior semicircular duct above. Surrounding these is a definite capsule of precartilage 

 tissue. 



FIG. 9. Sagittal section through the otic capsule in a human embryo 18 mm. long (Carnegie Collection, No. 144, slide 

 4, row 1, section 3). The section is 40/i thick and is enlarged 90 diameters. Above is the posterior semi- 

 circular duct, and just below the center is the lateral semicircular duct. The otic capsule is now differentiated 

 into relatively permanent areas of precartilage and other areas that are more temporary. The latter sur- 

 round the epithelial ducts and indicate the future cartilaginous canals. 



FIG. 10. Frontal section through the otic capsule in a human embryo 27 mm. crown-rump length (Carnegie Collection, 

 No. 756 a, slide 47, section 2). The section is 50M thick and is enlarged 90 diameters. It passes transversely 

 through the lateral semicircular canal. The epithelial duct is surrounded by a zone of temporary precartilage 

 corresponding to the future cartilaginous canal. Just median to the duct (below it in the photograph) is a 

 group of nuclei that forms the focus of the future growth of reticulum. 



FIG. 



epithelial duct, leaving a reticular area in the interval, which is more pronounced on the median side of tlir 

 duct (below it in the photograph). 



FIG. 12. Section through the lateral semicircular canal in a human fetus 37 mm. crown-rump length (Carnegie Collec- 

 tion, No. 972, slide 20, section 1). The section is 50^ thick and is enlarged 90 diameters. The nuclei of 

 the zone of temporary precartilage form a dark field that corresponds to the future cartilaginous canal. 

 Along the inner margin of this zone are seen large blood-vessels that belong to the periotic reticulum. 



FIG. 13. Section through the lateral semicircular canal in a human fetus 35 mm. crown-rump length (Carnegie Collec- 

 tion, No. 199, slide 58, section 2). The section is 50M thick and is enlarged 90 diameters. It is stained 

 deeply with hematoxylin, showing the matrix of the cartilage but not the zone of precartilage that is to become 

 the cartilaginous canal. 



PLATE 2. 



The figures on Plate II are in continuation of those on Plate I and show the final establishment of the periotic reticular 

 tissue. They also show, on being compared with younger stages, the manner in which the cartilage becomes 

 excavated in order to yield room for the enlarging duct and also to allow for its changing position. The 

 excavation is brought about by the dedifferentiation of cartilage into reticular tissue. Throughout this 

 period the margin of the cartilaginous canal continues in an unstable condition and is gradually either 



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