CONTAINED PERIOTIO TISSUE-SPACES IN THE HUMAN EMBRYO. 25 



This tinctorial reaction makes an arbitrary point at which it may be said that 

 precartilage becomes cartilage. 



All parts of the capsule do not take part in this process equally. It has already 

 been mentioned that during the period of differentiation of the precartilage tin- 

 tissue of the otic capsule loses its homogeneous character and some areas of it 

 begin to appear more dense than others. Immediately surrounding the semi- 

 circular ducts is quite a wide area of precartilage that appears less dense, which in 

 turn is inclosed by the main precartilaginous mass of the capsule whose nuclei give 

 it the appearance of greater density. This can be seen very well in figure 10. 

 When we come to embryos between 26 and 30 mm. long this contrast between the 

 two varieties of precartilage becomes more sharply denned, though the relative 

 compactness of the arrangement of the nuclei becomes reversed. The semicircular 

 ducts are then everywhere encircled by an area of temporary precartilage that 

 differs from the rest of the capsule and which is not to become true cartilage, but is 

 to be hollowed out to form the cartilaginous canals. This process of hollowing 

 out the cartilaginous spaces and replacing with reticular connective tissue the 

 precartilage that originally filled them forms a very interesting feature in the devel- 

 opment of the otic capsule, to which we will refer later. 



The difference between temporary precartilage and true cartilage is show'n 

 clearly in figure 11. This section passes transversely through the lateral semi- 

 circular canal of an embryo 30 mm. long (Carnegie Collection, No. 86). An area 

 of temporary precartilage surrounds the epithelial duct, forming a dark circular 

 field outside of which is the more permanent capsular mass. Examination under 

 higher powers shows that the temporary precartilage differs from the main mass 

 in that the nuclei are arranged somewhat concentrically, and there is less space 

 between them than exists in the latter, which is the reason for its darker appearance. 

 Furthermore, whereas the temporary precartilage around the semicircular ducts 

 retains the general histological features that were seen in the younger stages, the 

 main capsular mass has matured into well-defined cartilage. A specimen of about 

 1 his same age is shown in figure 13 (Carnegie Collection, No. 199, 35 mm. long). This 

 specimen was stained only in hematoxylin, which emphasizes the matrix. In such 

 a preparation the cartilaginous matrix is stained intensely blue, whereas the tem- 

 porary precartilage around the semicircular ducts takes the stain only in its nuclei. 

 The reverse picture is shown in figure 12, where the tissues show an intense nuclear 

 stain. This is taken from an embryo of about the same age as that shown in figure 

 13. Here, on account of the nuclei and the intervening dense protoplasm, the 

 temporary precartilage forms a dark mass around the semicircular duct. Figures 

 12 and 13 are like a positive and negative and approximately indicate the outlines 

 of the eventual cartilaginous canal. The area of temporary precartilage gradually 

 retracts towards the border of the more permanent cartilage, as we shall see in 

 the later stages, and as it does so the space becomes occupied by a reticulum of 

 connective tissue. 



In passing from embryos 30 mm. long to older stages, such as shown in figures 

 12, 13, and -14, the tissues show some advance in the degree of their maturation. 



