48 



ANATOMY 



less SO at 525 days. At three months a crest and fossa become 

 well developed across the lateral portions of the parietals. Both 

 are fully outlined at 40 days, but more evident at 54 days. They 

 are most conspicuous in the skull at 726 days. 



Scapula. There is little ossification in the acromion and 

 spine at 40 days, and still much cartilage in the coracoid process 

 at 40 days which is essentially fused with the scapula at 99 days. 



Pelvis. Mature rats show a marked pectineal eminence and 

 a preacetabular tubercle. These are recognizable at 46 days and 

 the tubercle at 40 days. Sutural grooves are recognizable at 

 the junctions of the ilium, pubis and ischium at 73 days, but 

 onlj" traces remain at 99 days. 



TABLE 15 

 Fusion of cartilage plates {Dawson, MS. '23) 



Fusion of epiphyses of the long bones. A. B. Dawson (manu- 

 script, '24) has made observations on the fusion of the epiphyses 

 in the various bones of rats one and two years of age. 



His results on the limb bones are given in table 15. Cartilage 

 plate present (P.), cartilage plate absent (A.). 



It is apparent from this that these bones are still in a condi- 

 tion to increase in length after two years, and this is in harmony 

 with the long continued skeletal growth as determined by direct 

 measurements. 



Additional information is given in table 15 A for the time of 

 fusion in the bones which fuse early, and also a set of observa- 

 tions on bones from a rat 37 months old. 



The proximal epiphysis of the ulna appears to be rather 

 variable. Since this is an extra-articular epiphysis it will not 



