THE EMBRYOLOGY OF THE OPOSSUM 195 



the hyoid arch at any time. As such a connection has been 

 described in a great many mammals this matter needs further 

 study, and I have referred the problem as well as the detailed 

 study of the development of all the ossicles to Mr. J. A. 

 McClain who is now working in this laboratory. Accordingly, 

 I shall content myself in this paper with a brief comment on 

 only two stages. 



Figure 61 shows the condition of the ossicles at birth. This 

 is a familiar arrangement and needs very little comment. 

 Meckel's cartilage extends from its articulation with the incus 

 all the way forward through the core of the mandible. The 



portion of 

 periot'ic 

 capsule 



caps 



incus 



Meek el 3 

 cartilage 



*- i,uw 4te-vai*. _ 



-stapes 



Fig. 61 Reconstruction of cartilaginous auditory ossicles at time of birth 

 (17175). Lateral view. 



stapes is penetrated by the stapedial artery, a branch of the 

 internal carotid. The hyoid cartilage is continuous with the 

 periotic capsule and makes no contact with the stapes, which 

 is medial and slightly anterior to it. 



Figure 62 shows a much later stage near the time of the 

 beginning of auditory function. This specimen was 49 days 

 of age (i.e., post partum), and was tested for auditory re- 

 flexes before he was killed. No reflexes were observed, though 

 in the light of later studies (McCrady, Wever and Bray, not 

 yet published) it seems likely that the first electrical responses 

 of the cochlea could be detected at this time. 



