204 EDWARD MCCRADY, JR. 



When the operations are performed less than 4 days after birth, 

 it is fairly common for the young to become detached and fall out of 

 the pouch. After about a week no such danger is encountered even 

 if the specimen dies, he remains attached to the nipple. From this 

 time until about 45 days after birth, if the young is forcibly removed 

 from the nipple, he is usually unable to re-attach, and so dies. After 

 about 45 days they can be removed without danger of tearing the lips, 

 as the mouth is nearly ready to open spontaneously, and when re- 

 placed in the pouch the young will resume nursing. 



In connection with studies of the development of hearing (done in 

 collaboration with several others) I have operated upon pouch young 

 between 48 and 80 days of age in order to place an electrode upon the 



round window 



para-occipital 

 process 



Fig. 65 Ventrolateral view of posterior part of adult skull (from McCrady, 

 Wever and Bray, '37 a). 



round window for the purpose of observing the Wever and Bray 

 phenomenon. The operative procedure is rather different from that 

 employed on placental mammals, because in the opossum the 

 squamosal, petrosal, and tympanic bones do not fuse to form a single 

 temporal bone, but remain permanently separate as in reptiles 

 (fig. 65). Accordingly, the middle ear is not encased in bone. In 

 adults prolonged muscular traction has caused the mastoid portion 

 of the squamosal bone to elongate and slightly overhang the round 

 window. For this reason, animals less than 100 days old are prefer- 

 able for this operation. 



Figure 66 shows the operative field in a specimen 78 days old. The 

 following summary of the operative procedure is taken from McCrady, 

 Wever and Bray ( '37) : 



