THE EMBRYOLOGY OF THE OPOSSUM 169 



extuberance the vestige of a horny bill which served the 

 ancestors of the opossum as a grasping organ. It can hardly 

 have any function in the opossum, since at the time of birth it 

 has already degenerated, and can not, therefore, participate in 

 tearing the embryonic membranes. 



This structure sometimes appears as early as the end of 

 the twelfth day and then persists through the early part of 

 the thirteenth. Similarly I have obtained one batch of 

 stage 33 embryos 11 days and 9 hours post coitum (i.e., before 

 the middle instead of after the middle of the twelfth day). 

 These last four stages are of rather long duration, and, of 

 course, like all other stages, they are not sharply demarcated 

 but grade into one another. I have placed them in the 

 temporal scale where they seem most typical. Some sources 

 for errors of a few hours in the timing of stages are mentioned 

 in the next section in connection with the discussion of the 

 length of gestation. 



The palate. Presumably as a result of active opening of 

 the mouth, the tongue drops below the level of the lateral 

 palatine processes which then come together in the midline 

 and fuse below the nasal septum. In their anterior third they 

 also touch the septum and fuse with it. In regard to the open- 

 ing of the mouth and the relation of the position of the tongue 

 to the lateral palatine processes, it should be mentioned that 

 though the mouth is open to some extent in stage 33 (see 

 frontispiece and fig. 49) the tongue never slips below the 

 palatine processes at that time. The tip of the tongue in the 

 frontispiece is projecting through the incisive foramen, the 

 body of the tongue lies dorsad of the palatine processes, the 

 frenulum of the tongue lies in the narrow slit between the 

 palatine processes. Only the tip of the tongue is seen in this 

 figure. In the posterior part of the angle between the lips 

 the lateral palatine process hides the body of the tongue from 

 view. The oblique line across the mouth is the anterior edge 

 of the palatine process. 



In stage 34 though the epitrichium has covered over the 

 sides of the mouth, which gives the impression that the jaws 



