THE EMBRYOLOGY OF THE OPOSSUM 205 



After anaesthetization with Avertin, injected intraperitoneally in 

 doses of 0.0001 cc. per gram of body weight a skin incision some 

 | inch long was made in the direction indicated in this figure. With 

 a long, slender, pointed scalpel the loose connective tissue behind the 

 facial nerve and immediately ventral to the mastoid process of the 

 squamosal bone was then trimmed away. The sterno-mastoid muscle 

 is continuous with the trapezius from their common origin on the 

 mastoid process down to the point where the great auricular nerve 

 comes between them. It was sometimes necessary, particularly in 

 the older specimens, to cut away the muscle tissue for a few milli- 

 meters at this common origin to expose the endodermal lining of the 

 middle ear. The exact point at which the endoderm was then cut is 

 caudal to the exit of the facial nerve, ventral and as close as possible 

 to the mastoid process, and dorso-cranial to the origin of the digastric 

 muscle. Little harm can come from cutting somewhat too far caudad. 



Fig. 66 Middle ear operation in 78-day-old opossum (from McCrady, Wever 

 and Bray, '37 a). 



as the paraoccipital process (fig. 65) limits digression in that direc- 

 tion ; but it is important not to cut too far craniad as the tympanum 

 and ossicles may be injured. The best limiting landmarks in the 

 cranial direction are the exit of the seventh nerve and a small tubercle 

 on the petrosal bone .... seen in figure 65 below and slightly to 

 the left of the round window. 



My former conjecture that this tubercle "probably represents the 

 styloid process of higer mammals" was quite wrong. The attach- 

 ment of the hyoid cartilage to the periotic capsule at the end of the 

 embryonic period (fig. 61) is dorsocaudal to the stapes and the 

 fenestra ovalis instead of ventrocranial to the fenestra rotunda, and 

 there is no indication of this tubercle at this time. The latter arises 

 much later as a process 'of the petrosal bone not associated in any 

 way with the hyoid cartilage. It provides a surface for the attach- 

 ment of the caudal border of the petrosal-alisphenoid ligament which 

 forms part of the ventral wall of the middle ear cavity. 



