36 



THE HISTOGENESIS AND GROWTH OF THE OTIC CAPSULE AND ITS 



narrow precartilage-reticular transitional zone between the cartilage and the peri- 

 chondrium. Presumably this indicates that the margin is still in an unstable 

 condition. 



^-^a^ Epithelium 

 -JC3^~c5S 



After the perichondrium has 

 made its first appearance it rap- 

 idly becomes thicker and more 

 conspicuous. In a fetus 80 mm- 

 crown-rump length (Carnegie Col- 

 lection, No. 172) it is found as 

 quite a dense fibrous coat, more 

 than twice as thick as that shown 

 in the 73 mm. embryo in figure 4. 

 It is clearly separated from the 

 cartilage and precartilage by a 

 narrow zone of reticular tissue. 



The character of the peri- 

 chondrium as existing in slightly 

 older fetuses is shown in figure 18, 

 which represents a section through 

 the posterior semicircular canal of 

 a fetus 85 mm. crown-rump length 

 (Carnegie Collection, No. 1400- 

 30). Here the perichondrium 

 consists of a relatively broad 

 zone of embryonic fibrous con- 

 nective tissue, which in the pho- 

 tograph is about 5 mm. wide, 

 encircling the whole canal. It 

 can be seen on the median side 

 (to the left) that it is separated 

 from the cartilage and adjacent 

 transforming precartilage zone by 

 a narrow, lighter area, which un- 



Membr. prop. 



- 



> Rctic u I um 



,^- Perichondrium 



Cartilaqe 



FIG. 4. Detail of the posterior canal in a human fetus 73 mm. long 

 (Carnegie Collection, No. 1373, slide 9, row 3. section 1) - 

 The section is 10/t thick and is enlarged 376 diameters. It 

 shows how the inner margin of* the reticulum becomes con- 

 densed into the membrana propria of the epithelial duct and 

 the outer margin into the perichondrium. The perichondrium 

 does not lie in direct contact with the cartilage, but is sepa- 

 rated by a narrow zone of tissue which consists of precartilage, 

 into which the cartilage is still being dedifferentiated, 



der higher magnification is found 

 to consist of reticular tissue. The membrana propria at the inner margin of the 

 reticulum is fairly well developed and it can be seen how it forms a supporting 

 coat to the epithelial duct. 



When one examines the cartilaginous semicircular canals in fetuses 130 mm. 

 long there can no longer be any question as to the identity of the perichondrium. 

 A specimen showing the superior semicircular canal at this stage is represented in 

 figure 19, which is taken from a fetus 130 mm. crown-rump length (Carnegie Col- 

 lection, No. 1018). The blood-vessels are injected with India ink. The main car- 

 tilaginous mass in this specimen is quite mature; the capsules are well defined and 

 the cartilage cells now possess a considerable amount of granular body-protoplasm. 



