SKELETON, MUSCULAR SYSTEM AND SKIN 821 



ossific points appear, one behind another. The posterior point of 

 ossification is for the basilar portion of the occipital bone, which is 

 developed in the same way as one of the vertebrae ; the middle point is 

 for the posterior portion of the sphenoid ; and the anterior point is for 

 the anterior portion of the sphenoid. The frontal bone, the parietal 

 bone, the temporal bone and a portion of the occipital bone are devel- 

 oped from the connective tissue without the intervention of preexisting 

 cartilaginous structure. At the time when the vertebrae are developed, 

 with their laminae and their spinous and transverse processes, the ribs 

 extend over the thorax, and the clavicle, scapula and sternum make 

 their appearance. 



At about the beginning of the second month, four papillary promi- 

 nences, the first traces of arms and legs, appear on the -body of the 

 embryo. These progressively increase in length, the arms appearing 

 near the middle of the' embryo, and the legs, at the lower portion. 

 Each extremity is divided into three portions, the arm, forearm, and 

 hand, for the upper extremities, and the thigh, leg and foot, for the 

 lower extremities. At the end of each extremity, there are, finally, 

 divisions into the fingers and toes, with the various cartilages and bones 

 of all these parts, and their articulations. 



Early in intra-uterine life the skeleton begins to ossify from bony 

 points in the cartilaginous structure. The first points appear at nearly 

 the same time about the beginning of the second month in the 

 clavicle and the upper and the lower jaw. Similar ossific points, which 

 gradually extend, are seen also in the other parts, the head, ribs, pelvis, 

 scapula, metacarpus and metatarsus, and the phalanges of the fingers 

 and toes. At birth the carpus is entirely cartilaginous, and it does not 

 begin to ossify until the second year. The same is true of the tarsus, 

 except the calcaneum and astragalus, which ossify just before birth. 

 The pisiform bone of the carpus is the last to take on osseous transfor- 

 mation, this occurring between the twelfth and the fifteenth years. As 

 ossification progresses, the deposits in the various ossific points grad- 

 ually extend until they reach the joints, which remain incrnsted with 

 the permanent articular cartilage (see Plate X, Fig. 3). 



While the skeleton is thus developing, the muscles are formed from 

 the outer layer of the mesoblast, and the visceral plates close over the 

 thorax and abdomen in front, leaving an opening for the umbilical cord. 

 The various tissues of the external parts, particularly the muscles, begin 

 to be distinct at the end of the second month. The deep layers of the 

 dorsal muscles are the first to be distinguished ; then successively, the 

 long muscles of the neck, the anterior straight muscles of the head, 

 the straight and transverse muscles of the abdomen, the muscles of the 



