300 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



(2) the vertebral border (crest) has become enormously elongated 

 and gives attachment to abdominal muscles, cutting off the fibres 

 of insertion of the external oblique which form the chief part of 

 Poupart's ligament ; (3) the ischium does not reach the ventral line. 

 In most birds, neither ischium nor pubes reach the ventral line. 

 The pubes fail to meet in cases of ectopia vesicae. The symphysis 

 pubis is formed in the ventral line. The cotyloid bone — os 

 acetabuli — is formed in the Y-shaped cartilage between the three 

 elements. It ossifies in the 13th year. Professor Howes has 

 pointed out that it is this ossification which forms the pubic 

 part of the acetabulum, and that it is really part of the pubes. 



Shoulder Girdle. — The duckbill (ornithorynchus) shows the 

 most generalized type of mammalian shoulder girdle ; it resembles 

 closely the primitive reptilian type ; from such a form the various 

 types of mammalian shoulder girdle were probably evolved. 



The dorsal part of the arch consists of (1) scapula, (2) supra- 

 scapula (Fig. 244). The supra-scapula is represented in man by 

 the cartilage along the vertebral border ; it ossifies in the early 



clavicle (dermal) 

 spine / j n f er clavicle (dermal) 



Fio. 244.— The Shoulder Girdle of Ornithorynchus. 



years of manhood. The supra-spinous part of the scapula appears 

 first in higher mammals ; it is produced late in the development 

 of the scapula (in the 3rd month of foetal life) by the upgrowth 

 of the supra-spinous blade of the scapula ; it is not represented 

 in the pelvic girdle. The spine of the scapula is represented in 

 the pelvic girdle by the ilio-pectineal line of the ilium ; but there 

 is no pelvic representative of the acromion process. The dorsal 



