638 EMBRYOLOGY. 



Furthermore the formation of the anterior limbs outstrips some 

 what that of the posterior. 



In the development of the skeleton of the extremities there are to be 

 recognised, as in the vertebral column and the skull, three different 

 stages, the stage of the membranous, that of the cartilaginous, and that 

 of the osseous fundament. 



After these general remarks I turn to the detailed description of 

 (1) the pectoral and pelvic girdles, (2) the skeleton of the appendage, 

 which projects free from the surface of the trunk, and (3) the 

 formation of joints. 



(a) Pectoral and Pelvic Girdles. 



The fundaments of the girdles of the limbs consist each of a pair 

 of curved pieces of cartilage, which are imbedded under the skin in 

 the muscles of the trunk, and which bear near the middle an articular 

 surface for the reception of the skeleton of the free extremity. By 

 this each cartilage is divided into a dorsal half, near the vertebral 

 column, and a ventral half. The former is converted in Mammals 

 and Man into a broad shovel-shaped piece ; the ventral half, which 

 reaches to, or nearly to, the median plane, is, on the contrary, 

 divided into two diverging processes, an anterior and a posterior. 

 The cartilaginous pieces thus distinguishable ossify from special 

 centres, and thereby acquire a higher degree of independence. 



The shoulder-blade (scapula) of Man is at first a cartilage of a 

 form similar to that of the adult, except that the basis scapulae is 

 less developed. In the third month ossification begins at the collum 

 scapulae. However, the margins, the spine, and the acromion 

 remain for a long time cartilaginous, and indeed are in part so even 

 at the time of birth. There arise in them here and there accessory 

 centres during childhood. 



From the articular part of the shoulder-blade there runs ventrally 

 a cartilaginous process, which is short in Man, but in other Verte- 

 brates is of considerable size and reaches down to the sternum. It 

 corresponds to the posterior of the previously mentioned diverging 

 processes into which the ventral part of the cartilaginous arch is 

 divided, and is known in comparative anatomy as pars coracoidea. 

 In Man it is only slightly developed. Its great independence, however, 

 is made evident by its acquiring in the first year after birth a sepa- 

 rate centre of ossification. From this there gradually arises a bony 

 element (os coracoideum), which is joined to the shoulder-blade until 



