EXCAVATION OF THE CARTILAGINOUS OTIC CAPSULE 5 



around the otic vesicle; secondly, the differentiation of the con- 

 densed mesenchyme into precartilage; and thirdly, the conver- 

 sion of precartilage into true cartilage. These three histogenetic 

 stages merge more or less diffusely into one another and one 

 must bear in mind that such a subdivision is necessarily, 

 arbitrary and tends to result in an exaggeration of the distinct- 

 ness of the lines of their demarcation. Their points of dif- 

 ference, however, are here emphasized because the reversal of 

 one state of development into a previous state is the feature to 

 which it is desired to call especial attention. 



STAGE OF CONDENSED MESENCHYME 



When a human embryo is 4 to 5 mm. long the mesenchy- 

 mal tissue surrounding the otic vesicle differs very little from 

 that in other regions. The nuclei, however, are quite sparse 

 in the regions ventral to the neural tube in the median line, and 

 they become perceptibly more numerous as one explores later- 

 ally into the neighborhood of the otic vesicle. This sHght in- 

 crease in the number of nuclei around the vesicle marks the 

 beginning of the mesenchymal condensation that is to form the 

 otic vesicle. A definite layer of such nuclei is not found until 

 the embryo reaches a length of about 9 mm. ; it is then possible 

 to recognize a fairly well outlined zone of mesenchyme which 

 represents the otic capsule in its first stage of development. 

 In figure 2 is shown a sketch indicating the relations which 

 exist at that time. It represents a transverse section through 

 the otic vesicle at the level of the attachment of the endolym- 

 phatic appendage. The zone of condensed mesenchyme forming 

 the primordium of the otic capsule abuts directly against the lat- 

 eral wall of the vesicle and extends from there to a point about 

 one-half the distance between the vesicle and the ectoderm. On 

 the median side of the vesicle this zone is lacking, although there 

 is a considerable number of mesenchyme cells clustered around 

 the vascular plexus ensheathing the central nervous system, and 

 among the nerve rootlets of the acoustic complex. When this 

 zone is analyzed under higher magnification it is found that it 

 still consists essentially of a mesenchymal syncytium. It differs 



