EXPLANATION OF PLATES. 53 



receding or advancing, through the processes of dedifferentiation, into precartilage or differentiation from 

 precartilage respectively. Tlie periotie reticulum in its later stages develops fibrous membranes at, its inner 

 :uid outer borders. Tin- one :it the inner border forms the meiubnuia pro]>riii for the epithelial duct, and the 

 one at the outer border becomes the pcrichondrium. 



FIQ. 14. Section through the lateral semicircular canal in a human fetus 43 mm. crown-rump length (Carnegie Col- 

 lection, No. 886, slide 12, section 3). The section is 100M thick and is enlarged 90 diameters. The zone of 

 precartilage is expanding around its peripheral margin by dedifferentiation of the surrounding cartilage and 

 on its central margin the precartilage is giving way before the advancing reticulum. A crescentic area of 

 periotie reticulum is established on the median side (to the left) of the epithelial duct, about X mm. deep in 

 the photograph. 



I 1 ui lf>. Section through the lateral semicircular canal in a human fetus 46 mm. crown-rump length (Carnegie Collec- 

 tion, No. 95, slide 72, section 1). The section is 100M thick and is enlarged 90 diameters. The original area 

 of precartilage is now all dedifferentialed into reticulum, and a new area of precartilage has formed outside 

 of this at the expense of the surrounding cartilage. The new area of precartilage is about 0.8 cm. deep in the 

 photograph. Everything between this and the epithelium is reticulum, the peripheral part of which is not, 

 yet completely vascularized. 



FIG. Iti. Section through the posterior semicircular canal in a human fetus 50 mm. crown-rump length (Carnegie 

 Collection, No. 184, slide 23). The section is 50^ thick and is enlarged 90 diameters. The dedifferentiation 

 of precartilage into reticulum is nearly complete, there being left only a narrow line of it along the margin of 

 the cartilage. The vascularization of the reticulum is not yet completed. The small diameter and the thick 

 wall of the epithelial duct in this figure and in figure 15 result from contraction. If they were distended in 

 the process of fixation they would doubtless be as large as those in figures 14 and 17. 



FIG. 17. Section through the posterior semicircular canal in a human fetus 52 mm. crown-rump length (Carnegie 

 Collection, No. 96, slide 12, section 2). The section is 100/n thick and is enlarged 90 diameters. It differs 

 from figure 10 in having a more mature periotie reticulum. 



Fiu. IS. Section through the posterior semicircular canal in a human fetus, 85 mm. crown-rump length (Carnegie 

 Collection, No. 1400-30, slide 43, section 2). The section is lOOju thick and is enlarged 90 diameters. At 

 the inner margin of the reticulum can now be seen the membrana propria supporting the semicircular duet 

 and at the outer margin is the thick perichondrium, between which and the cartilage there is a narrow open 

 space that is better seen on the left part of the photograph. The sharp dark line along the margin of the 

 cartilage on the right is an appearance due to the excavation of cartilage at that point. It consists of an 

 intermediate zone in which the cartilage is being dedifferentiated into precartilage and that in turn into 

 reticular tissue. 



Fiu 1!).- Section through the superior semicircular canal in a human fetus 130 mm. crown-rump length (Carnegie Col- 

 lection, No. 1018, slide 30, section 1). The section is 50/n thick and is enlarged 90 diameters. It shows a 

 rather mature perichondrium closely attached to the cartilage, separated from it, however, by a narrow 

 intermediate zone that is not seen in tire photograph. This zone is connected with the further enlargement 

 of the cartilaginous canal, the growth of which is'not yet completed. In the outer part of the canal the 

 perichondrium fuses with the membrana propria of the semicircular duct. The periotie reticulum is 'begin- 

 ning to break up in the formation of larger spaces, which it does by the retraction of its trabeculse, thereby 

 allowing adjacent spaces to coalesce. The blood-vessels in this specimen were injected with India ink. 



PLATE 3. 



The figures on Plate III show the histological appearance of the periotie tissue-spaces and the manner in which they 

 are formed from the periotie reticulum. This is accomplished by the disappearance of the trabecula 1 and the 

 consequent repeated coalescence of adjoining spaces. 



FIG. 20. Section through the second turn of the cochlea in a human fetus 130 mm. crown-rump length (Carnegie 

 Collection, No. 1018, slide 32, section 2), enlarged 57 diameters. This section shows the topography of the 

 cochlear duct and the general character of the periotie spaces that are developing along its inner margins. 

 Details of this same section as seen under higher magnification are shown in figures 22 and 24. 



Fiu. 22. Detail of the section shown in figure 20, enlarged 278 diameters. This figure shows the part of the cochlear 

 duct that is to form the organ of Corti with the adjacent tissue that becomes incorporated in the basilar mem- 

 brane. Below is the periotie reticulum, whose spaces are in the process of enlarging. By repeated coales- 

 cence these spaces finally unite with the large space which constitutes the scala tympani. This figure shows 

 the histological appearance of the reticulum where the formation of tissue-spaces is in active operation. 



Fiu. 24. Detail of the section shown in figure 20, enlarged 300 diameters. It shows the character of the margin of the 

 scala vestibuli in a fairly mature condition. The scala vestibuli is inclosed by a membrane consisting of the 

 cells that had previously constituted the reticulum occupying this area and which have been modified in form 

 in adaptation to the formation of this large tissue-space, closing it off from the surrounding tissue. 



Fin. 21. Section through the vestibular portion of the labyrinth in a human fetus 52 mm. c.rown-iump length (Carnegie 

 Collection, No. 448, slide 154, section 2), enlarged 31 diameters. This section shows the general character 

 of the periot ic spaces and their relation to the different parts of the membranous labyrinth and the surround- 

 ing cartilaginous capsule. The first space to develop and the largest shown in this figure is the vestibular 

 cistern, situated between the utricle and the cartilaginous stapes. The smaller spaces, below the cistern 

 and extending downward along the cochlear duct, represent the scala vestibuli in an early form The 

 arteries in this specimen were injected with India ink and are shown in black. 



