CONTAINED PERIOTIC TISSUE-SPACES IN THE HUMAN EMBRYO. 29 



spicuously at the junction of the cartilage with the precartilage. Its wavy refrac- 

 tive lines are so compact that under low powers the whole zone appears as a dark 

 rim outlining the cartilaginous margin of the canal. The compactness of these 

 lines varies in different embryos of about the same age and even varies in the two 

 borders of a given canal. 



This latter condition can be seen in figure 14, which represents the lateral semi- 

 circular canal of a fetus 43 mm. crown-rump length (Carnegie Collection, No. 886). 

 It will be noted that the peripheral two-thirds of the intermediate zone (toward the 

 right hand) forms a dark, heavy margin between the true cartilage and the encircled 

 precartilage, whereas the central one-third (toward the left hand) is wider and much 

 less distinct. It can also be seen that the place at which this intermediate zone is 

 well marked corresponds to the direction of the excavation necessary to allow for the 

 growth of the canal and to make room for the elongating semicircular ducts of the 

 contained membranous labyrinth. In this case the expansion must be toward the 

 periphery of the cartilaginous capsule, i.e., toward the right side of the photograph. 

 From studying various fetuses it seems to be true that where excavation of cartilage 

 is actively going on at such a place there is found a prominent intermediate zone along 

 the inner margin of the cartilage. Sometimes the line is uniform around the entire 

 rim, but usually it is more marked on one side of the canal than on the other, and 

 in such cases it is always toward the direction of the excavation of the cartilage, 

 as can be judged from the topography of the labyrinth. 



If an older specimen is examined, such as the one represented in figure 15, the 

 character and relative position of the cartilage and precartilage are found to be the 

 same as in the 30-mm. stage just described. They have, however, undergone an 

 alteration to allow for the enlargement of the cartilaginous canal. Figure 15 shows 

 the lateral canal in a human fetus about 50 mm. crown-rump length (Carnegie 

 Collection, No. 95). The fetus is catalogued as being 46 mm. long, but this is 

 apparently the slide-measurement. In its development it corresponds to fetuses 

 50 mm. long, formalin measurement, and this measurement is used so that it will 

 accord with the other fetuses. Since figures 11 and 15 represent sections through 

 the same canal taken at about the same place and under the same enlargement, one 

 can superimpose them, one upon the other, and thus determine the change that 

 has occurred between the two stages. If this is done it will be seen that the area 

 that was precartilage in the 30-mm. stage is replaced by reticulum in the 50-mm. 

 stage. There is just as much or more precartilage in the latter, but it has moved out- 

 ward into the area that was previously true cartilage. In other words, the enlarge- 

 ment of the cartilaginous canal has been obtained by a process of excavation based on 

 the dedifferentiation of true cartilage into precartilage and the latter in turn into 

 reticulum. This is shown under higher magnifications in text-figures 2 and 3, 

 which show sections of these same canals under the same enlargement and placed 

 side by side for the purpose of better comparison. It can be seen in these two 

 figures how the cartilage of 30-mm. stage becomes dedifferentiated into the pre- 

 cartilage of the 50-mm. stage and the border along which this process is in active 

 operation forms the intermediate zone, which is characterized by its wavy, refractile 



