396 GENERATION. 



prolongation of the chorda dorsalis, are two cartilaginous 

 processes, which are developed into the so-called cranial ver- 

 tebrae. In this cartilaginous mass, three ossific points ap- 

 pear, one behind the other. The posterior point of ossifica- 

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

 developed in the same way as one of the vertebrae ; the mid- 

 dle 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 developed from the 

 connective tissue, without the intervention of preexisting 

 cartilaginous structure. The development of the face will 

 be described separately. At the time when the vertebrae 

 are being developed, with their laminae and their spinous 

 and transverse processes, the ribs extend over the thorax, 

 and the clavicle, scapula, and sternum make their appear- 

 ance. 



At about the beginning of the second month, four papil- 

 lary prominences, which are the first traces of the arms and 

 legs, appear on the body of the embryon. These progress- 

 ively increase in length, the arms appearing near the middle 

 of the embryon, and the legs, at the lower portion. Each ex- 

 tremity 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 of these parts, and their 

 articulations. 



Very early in intra-uterine life, the skeleton, which is at 

 first entirely cartilaginous, begins to ossify, from little bony 

 points which appear 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 also 

 seen in other parts, the head, ribs, pelvis, scapula, metacarpus 

 and metatarsus, and the phalanges of the fingers and toes. At 



