26 THE HISTOGENESIS AND GROWTH OF THE OTIC CAPSULE AND ITS 



Their intense stain-reaction causes the area of temporary precartilage to stand out 

 very conspicuously. On examining under higher powers the section shown in 

 figure 12 (Carnegie Collection, No. 972, 37 mm. long), it is seen that the nuclei in 

 the precartilage area are somewhat more numerous and are more compactly 

 arranged than in the same area in figure 1 1 . The darker appearance as contrasted 

 with the surrounding cartilage is also due partly to the fact that the compact mass 

 of internuclear protoplasm is distinctly tinged by the acid stains, whereas in the 

 surrounding permanent cartilage the matrix is nearly devoid of any color, having 

 been decolorized by the differential stain. 



In addition to the staining reaction there is now a marked difference in struc- 

 ture between the more permanent cartilage and the temporary precartilage. The 

 latter retains its precartilaginous character. Its more peripheral cells show a slight 

 tendency to capsule-formation. A common form among these is an oblong nucleus 

 with thickened elongated processes at the four corners, resembling the pronged 

 egg-case of the shark, the spaces between the processes on each side of the nucleus 

 being parts of the incomplete capsular space. These cells are arranged in circular 

 lines parallel with the circumference of the canal. The transition into true carti- 

 lage is rather abrupt, and on advancing into this region one meets with a character- 

 istic matrix, embedded in which are the completely encapsulated nuclei. The 

 temporary precartilage in its more central layers, near the reticulum, does not 

 show any tendency towards encapsulation. Its nuclei are arranged in concentric 

 layers with a small amount of compact protoplasm between them, resembling an 

 early stage of fibrous connective tissue. 



A layer of blood-vessels marks the junction of the temporary precartilage with 

 the reticulum surrounding the semicircular duct. This reticulum appears lighter 

 than the surrounding precartilage because of the free spaces between its slender 

 trabeculae. Furthermore, the nuclei are not quite so numerous and are more 

 irregularly arranged. The reticulum does not advance very rapidly in its develop- 

 ment, and it is not until we come to embryos between 40 and 50 mm. long that 

 we meet with an extensive reticulum. The development of this reticulum will 

 be described after we have taken up some of the subsequent changes in the cartilage. 



GROWTH AND ALTERATION OF FORM OF THE CARTILAGINOUS CANALS. 



In embryos 30 mm. long the cartilaginous labyrinth has attained approximately 

 the adult form. Its subsequent development is primarily an increase in size to 

 accommodate the growing membranous labyrinth. If a cast of the superior carti- 

 laginous canal of an 80 mm. embryo be compared with a similar cast of the same 

 canal in a 30 mm. embryo, it will be seen that the general form of the canal in the 

 older specimen is much the same as in the younger specimen But its diameter 

 and length have both increased, the diameter being nearly doubled and the length 

 trebled ; furthermore, its linear curvature corresponds to an arc with a considerably 

 longer radius. In reality, therefore, the developing cartilaginous labyrinth is con- 

 tinually undergoing changes, both in size and form. The histological evidence of 

 these changes constitutes one of the most interesting features in the development of 



