640 EMBRYOLOGY. 



expense of the cartilage, of which, however, considerable remnants 

 are still present at the time of birth. For the whole crest of the 

 ilium, the rim and fundus of the acetabulum, and the whole tract 

 from the tuberosity of the ischium to the spine of the pubis is still 

 cartilaginous. 



After birth the growth of the three bony pieces advances toward 

 the acetabulum, where they all meet, being however separated, up to 

 the time of puberty, by strips of cartilage, which together form a 

 three-rayed figure. At about the eighth year both the ascending 

 and descending rami of pubis and ischium fuse with each other, so 

 that at this time each hip-bone consists of two pieces joined by 

 cartilage at the acetabulum the ilium and an ischio-pubic bone. 

 These do not become united into one piece until the time of puberty. 



As in the pectoral girdle, so also in the pelvic girdle, there occur 

 accessory centres of ossification ; of these one, which sometimes arises 

 in the cartilage of the acetabulum, is the most important, and is 

 described as os acetabuli. Others arise in the cartilaginous crest of 

 the ilium, in the spines and tubercles, and in the tuberosity of the 

 ischium. They are not united with the chief bones until the end of 

 the period of growth. 



(b) Skeleton of the Free Extremity. 



All skeletal parts of the hand, fore-arm, and arm, as well as of the 

 foot, leg, and thigh, are originally solid pieces of hyaline cartilage, 

 which early acquire the general forms of the bones that subsequently 

 replace them. They are marked off from their surroundings by a 

 special fibrous layer of connective tissue, the perichondriuin. 



From the beginning of the third month the process of ossification 

 takes place in the larger skeletal pieces, by means of which the 

 cartilaginous tissue is destroyed and replaced by osseous tissue, in the 

 same manner as in the vertebral column. In this process several 

 general phenomena regularly make their appearance ; I shall go 

 somewhat into the details of these, without however taking into 

 account the complicated histological changes, information concerning 

 which is given in text-books of histology. 



The process of ossification takes externally a somewhat different 

 turn according as the cartilages are small and uniformly developed 

 in all directions, as in the wrist and ankle, or have become more 

 elongated. 



In the first case the course of development is more simple. From 



