THE FACTORS INVOLVED IN THE EXCAVATION OF 



THE CAVITIES IN THE CARTILAGINOUS CAPSULE 



OF THE EAR IN THE HUMAN EMBRYO 



GEORGE L. STREETER 



Department of Embryology, Carnegie Institution of Washington, Baltimore, 



Maryland 



TWELVE FIGURES 



The main mass of the cartilaginous capsule of the ear matures 

 into true cartilage when the human embryo reaches a length 

 of 20 to 30 mm., at which time it has acquired what may be 

 considered its adult form with characteristic chambers and 

 openings. From this time on, throughout its whole cartilag- 

 inous period, and even after ossification has begun, it undergoes 

 continuous growth, maintaining at the same time, however, 

 its general form and proportions. Such a growth involves 

 both an increase in the surface dimensions of the capsule and a 

 gradual enlargement or excavation of its contained cavities. It 

 is to the manner in which this excavation is accomplished that 

 the T\Titer wishes to call attention and particularly to the factors 

 concerned in its progress whereby a suitable space is always 

 pro\'ided for the enlarging membranous labyrinth. The actual 

 amount of increase in size of the labyrinth is graphically pic- 

 tured in figure 1. The outlines are made so that they show on 

 the same scale of enlargement a series of wax-plate models of 

 the left membranous labyrinth of human embryos having a 

 crown-rump length of 20, 30, 50, 85 and 130 mm., as indicated 

 in the figure. This covers the period during which the otic 

 capsule is in a cartilaginous state. Ossification begins when the 

 fetus has attained a crown-rump length of about 130 mm. The 

 growth from then until the adult condition is reached may be 

 judged by comparing the above with the final stage, labelled 



1 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 22, NO. 1 

 JULY, 1917 



