22 THE HISTOGENESIS AND GROWTH OF THE OTIC CAPSULE AND ITS 



found in the same embryo in the ordinary mesenchyme in that neighborhood, but 

 are less numerous. This condensed tissue differs in one respect quite definitely 

 from ordinary mesenchyme, in that it is almost devoid of blood-vessels, excepting 

 along its margins. To all appearances it abuts, as in younger specimens, directly 

 against the epithelial wall of the labyrinth. 



DIFFERENTIATION OF PRECARTILAGE. 



The histogenetic changes which mark the beginning of the conversion of the 

 condensed mesenchyme into a cartilage-like tissue make their first appearance just 

 after the separation of the semicircular ducts from the main vestibular pouch. 

 This occurs in embryos about 14 mm. long. In embryos about 30 mm. long the 

 otic capsule has the appearance and gives the tinctorial reactions of true cartilage. 

 Thus, in embryos between 14 mm. and 30 mm. long, the otic capsule consists of a 

 tissue that is intermediate between a condensed embryonic connective tissue and 

 cartilage, and this intermediate form is known as precartilage. 



The appearance of the otic capsule just at the time the canals are forming is 

 shown in figure 8, which is from an embryo 15 mm. long (Carnegie Collection, No. 

 719). The section passes horizontally through the labyrinth. A portion of the 

 utricle is shown at the bottom of the photograph, and detached from it, above, is 

 the superior semicircular duct. A streak extending from the duct to the utricle 

 still persists. This streak represents the wall of the labyrinth that formerly occu- 

 pied this place and is now absorbed close up to the inner margin of the duct. 

 Surrounding the capsule is a plexus of blood-vessels. 



On examination under higher magnifications it is found that the tissue forming 

 the capsule at this time differs very little from the condensed mesenchyme which 

 we have seen in the younger stages. The most noticeable difference is that the 

 nuclei are beginning to stand more apart from each other. This can be seen by 

 comparing figures 7 and 8. In the former the section is 10/x thick, in the latter the 

 section is 40^ thick. In spite of being four times thicker, the section of the older 

 specimen shows only about the same number of nuclei that are seen in the thinner 

 and younger specimen in figure 7. 



Between the nuclei there are numerous branching slender processes. The 

 spaces between the processes are not as clear as the spaces in the adjoining sub- 

 cutaneous connective tissue, but contain a homogeneous substance that stains very 

 slightly with such a dye as alum cochineal. The accumulation of this substance is 

 doubtless related to the spreading apart of the nuclei and to the alteration in the 

 branching processes that begins to show at this time. In certain regions the pro- 

 cesses between the nuclei become less branched. Larger ones become more 

 prominent and the smaller ones begin to disappear. A common arrangement is 

 to find two or more larger processes uniting to form a loop at the side or at one or 

 both ends of the nucleus. This feature is characteristic of precartilage. There 

 is very little tendency as yet to an accumulation of denser protoplasm around 

 the nuclei. 



