THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 639 



the seventeenth year by a strip of cartilage, and may therefore be 

 detached. Afterwards it is united with the scapula by bony substance 

 and constitutes the coracoid process. Still later the fusion of the 

 accessory centres previously mentioned takes place, to which, how- 

 ever, no great morphological importance attaches. 



There are two different views concerning the place which the 

 clavicle takes in the shoulder-girdle. 



According to GOETTE, HOFFMANN, and others, it belongs to the 

 primordial skeletal parts, which are preformed in cartilage, and 

 corresponds to the anterior ventral process, which was present in the 

 primitive form of the shoulder-girdle. According to GEGENBAUR it 

 is a covering bone which has entered into union with the cartilaginous 

 skeleton in the same way as the covering bones of the skull have 

 with the primordial cranium. 



It is the peculiar method of the development of the clavicle that 

 has caused this divergence of opinion. This is the first bone to be 

 formed in Man ; it begins to be ossified as early as the seventh week. 

 The earliest bony piece, as GEGENBAUR was the first to ascertain, is 

 developed out of wholly indifferent tissue. Then there are added at 

 both ends masses of cartilage, which are softer and provided with 

 less intermediate substance than the ordinary embryonic cartilage. 

 They serve, as in other bones that are preformed in cartilage, for the 

 elongation of the clavicle at both ends. There is also developed in 

 the sternal end, between the fifteenth and twentieth years, a kind of 

 epiphysial centre, as KOLLIKER states ; this fuses sometimes as late 

 as the twenty-fifth year with the main piece. 



The original conditions are the most faithfully preserved in the 

 pelvic girdle, even in Man and Mammals. The first fundament of 

 the girdle consists of a right and a left pelvic cartilage, which are 

 united ventrally in the symphysis by means of connective tissue, and 

 each of which has at its middle an articular fossa. Each pelvic 

 cartilage is composed of an expanded part extending dorsally from 

 the articular depression, the iliac cartilage, which is joined to the 

 sacral region of the spinal column, and two ventral cartilaginous 

 rods, pubis and ischium, which, meeting in the symphysis, enclose 

 the foramen obturatorium. 



It is stated by ROSENBERG that the pubic cartilage is at first 

 formed independently, but that it soon fuses with the other cartilages 

 at the acetabulum. 



Ossification begins at the end of the third month in three places, 

 and thus are formed a bony ilium, os pubis, and ischium at the 



