CONTAINED PERIOTIC TISSUE-SPACES IN THE HUMAN EMBRYO. 35 



The first and relatively the major part of the hollowing-out of the cartilaginous 

 canals is complete before the perichondrium makes its appearance. This is illus- 

 trated, for instance, by the fetus of 52 mm. crown-rump length, in figure 17, where 

 there is as yet no indication of it shown. In fetuses between 40 and 50 mm. long 

 the zone of precartilage surrounding the margins of the canals, as seen in figures 

 14 and 15, might be mistaken for perichondrium. This area, however, in fetuses 

 slightly older is converted almost entirely into reticulum. Kolliker (1879), in the 

 second edition of his text-book on embryology, pictures a transverse section 

 through the lateral canal of a rabbit embryo (fig. 457, page 735), in which this 

 zone of precartilage is labeled as periosteum of the future bone. 



The real perichondrium does not make its appearance until the fetus reaches a 

 a length of about 70 mm. A specimen of this age is represented in text-figure 4, 

 which shows a segment of the posterior semicircular canal in a fetus 73 mm. crown- 

 rump length (Carnegie Collection, No. 1373). On examination of this specimen it is 

 found that there is a distinct condensation of the reticulum along its inner margin, so 

 that it forms a membrana propria for the epithelial duct with which it is in contact. 

 This area has largely lost its reticular character and now resembles embryonic 

 fibrous connective tissue. Along the outer margin of the reticulum a similar con- 

 densation of its trabeculse has taken place, forming a thin fibrous lamina or mem- 

 brane near the margin of the cartilage. This is the perichondrium in its early 

 form. It does not abut directly against the cartilage, but is separated from it by 

 a thin layer of transition tissue that is in process of dedifferentiation from pre- 

 cartilage into reticulum. 



Passing inward from the cartilage, the transitions are rapid from cartilage to 

 precartilage, from precartilage to the tissue that is in transition to the reticulum and 

 then to the perichondrium. These are found as narrow zones that merge quickly 

 from one into the other. One should remember that the cartilaginous canal has not 

 reached its full size yet, and that the margin of the canal is still in an unstable con- 

 dition. However, as the canal becomes larger and the tissues more mature, it is 

 found that the transitions between the different zones become more abrupt and 

 in this process the precartilage zone becomes relatively much narrower. This can 

 be seen by comparing text-figures 3 and 4. The width of the reticulum in these 

 two figures can not be compared, because they represent different canals, lateral 

 and posterior, and no attempt was made to take them from the same relative posi- 

 tions. The fact that the reticulum is narrower in figure 4 has no significance in 

 the question of growth. The wide precartilage zone in figure 3 as compared with 

 that in figure 4, on the contrary, has a direct bearing on the relative age of the two 

 specimens. A relatively wide zone of precartilage is characteristic of younger 

 stages. After fetuses become 70 mm. long the precartilage zone becomes quite 

 narrow, so that the transition from cartilage to perichondrium is relatively abrupt. 

 In older specimens one might easily obtain the impression that the perichondrium 

 rested directly against the cartilage, as doubtless it does in the adult. In the oldest 

 fetus examined, 130 mm. crown-rump length, there is still found a distinct though 



