CONTAINED PERIOTIC TISSUE-SPACES IN THE HUMAN EMBRYO. 31 



laginous vestibule increases, producing relatively a lateral migration of the space 

 as a whole. Such a migration involves the excavation of the established cartilage 

 on its lateral margin and the formation of new cartilage on its median margin. On 

 its lateral margin true cartilage is being dedifferentiated into precartilage and on 

 its median margin precartilage is differentiating into cartilage. It is this pheno- 

 menon that determines the conditions shown in figure 14. On the right can be 

 seen the prominent intermediate zone, indicating an active excavation of cartilage, 

 and on the left the line of transition between cartilage and precartilage presents 

 the same picture as that seen in the stage of differentiation of the latter into the 

 former. One is forced to conclude that the cartilaginous tissue of the otic capsule 

 is capable of differentiation and dedifferentiation in its earlier stages, at least up to 

 the time of the completion of the encapsulation of the cartilage cells. This pro- 

 gressive and retrogressive adaptability of the cartilaginous tissue makes possible the 

 changes that are necessary in the growth and alteration in form of the labyrinth. 



DEVELOPMENT OF PERIOTIC RETICULAR CONNECTIVE TISSUE. 



The formation of the connective-tissue reticulum surrounding the semicircular 

 ducts is first indicated by a cluster of darkly stained nuclei that lie along the central 

 edge of the ducts in embryos soon after the ducts are formed and before the differ- 

 entiation of the cartilage is completed. In figure 9 such a cluster is seen just under 

 the posterior duct in the upper part of the photograph. In figure 10, which shows 

 the lateral semicircular duct of an embryo 27 mm. long (Carnegie Collection, No. 

 756a), a similar cluster of nuclei can be seen just under the duct, in reality just 

 median to it. These foci mark the points at which the formation of the reticulum 

 begins. It is not, however, until we come to embryos about 30 mm. long that we 

 find a definite reticulum. At that time, as is shown in figure 11, a narrow lighter 

 area can be made out, situated between the epithelial wall of the duct and the 

 temporary precartilage. It is the development of this area at the expense of the 

 temporary precartilage that results in the reticulum in which the periotic spaces are 

 subsequently formed. This area consists of a mesenchymal syncytium containing 

 irregularly shaped clear tissue spaces and is characterized by the presence of numer- 

 ous blood-vessels and connecting capillaries. The larger vessels are found resting 

 against the inner margin of the temporary precartilage. They sometimes indent it, 

 but never penetrate it to any extent. Such vessels can be seen in figures 11 and 12. 

 The presence of these blood-vessels is coincident with the appearance of the retic- 

 ular tissue. 



In describing younger stages the statement has been made that the temporary 

 precartilage abuts directly against the epithelial wall of the semicircular duct. 

 This statement is based only on the gross appearance. On careful scrutiny of the 

 tissue that immediately surrounds the ducts in embryos between 14 mm. and 20 

 mm. long a few mesenchymal cells can be found which possibly do not belong to 

 the temporary precartilage. These cells may very well represent some of the 

 indifferent mesenchyme, and possibly also some angioblasts. It is conceivable 

 that these surround the otic vesicle in its earliest stages and are inclosed along with 



