BONES 



47 



Apparently the rate, time, and order of ossification are remark- 

 ably constant in the albino rat. Though only one Utter was 

 used for each stage, comparisons of successive stages and the 

 constancy in Utters seem to warrant this statement. 



TABLE 14a 



Comparative table giving time of first ossification of the skull bones in the rat and 

 man, during fetal life. (R. M. Strong, MS., '2Jf) 



Mandible 



Maxilla 



Frontal 



Squamosal 



Premaxillary. . . 



Parietal 



Basioccipital. . . 

 Exoccipital .... 



Vomer 



Palatine 



Lacrimal 



Nasal 



Tympanic 



Basisphenoid. . . 

 Interparietal. . . 

 Supraoccipital. 



Alisphenoid 



Orbitosphenoid 



17 days + 

 17 days + 

 17 days + 

 17 days + 

 17 days + 

 17 da5's + 

 17 days + 

 17 days + 

 17 days + 



17 days + 



18 days 

 18 days 

 18 days 

 18 days 

 18 days + 

 18 days + 

 18 days + 

 22 days + 



55 minutes 

 55 minutes 

 55 minutes 

 8i hours 

 8| hours 

 8j hours 

 8j hours 

 8| hours 

 8 J hours 

 8t hours 



9f hours 

 9§ hours 

 9| hours 

 19 hours 



39 days 

 39 days 

 56 days 

 56 days 

 42 days 

 56 days 

 65 days 



56 days 



57 days 

 57 days 

 85 days 

 57 days 

 65 days 

 83 days 

 57 days 



55-56 days 

 83 days 

 83 days 



Skull. Sutures: The following bones are more or less dis- 

 tinctly outlined by sutures at 525 days. 



Paired: Maxillae, premaxillae, nasals, palatines, frontals, 

 parietals, jugals, petrosals, squamosals and tympanies. The 

 angle of the nasals with the frontals becomes more marked in 

 old rats, giving a ''Roman nose" effect. 



Unpaired : The interparietal, supraoccipital, basisphenoid, and 

 presphenoid are all more or less distinct in the same specimen. 

 The interparietal is entirely outlined. The supraoccipital is 

 fused with the exoccipital. 



The dorsal surface of the skull of very old rats tends to be- 

 come roughened. This is conspicuously so at 726 days, and 



