DEVELOPMENT OF THE SKELETON. 817 



the chorda dorsalis which is surrounded by the bodies of the vertebrae disap- 

 pears, leaving the enlargements between the vertebrae distinct. These en- 

 largements, which are not permanent, are gradually invaded by fibrous tis- 

 sue, their gelatinous contents disappear, and the intervertebral disks, com- 

 posed of fibro-cartilaginous structure, remain. These disks are permanent 

 between the cervical, the dorsal and the lumbar vertebrae ; but they eventu- 

 ally disappear from between the different parts of the sacrum and coccyx, as 

 these are consolidated, this occurring, in the human subject, between the 

 ninth and the twelfth years. 



Vertebral Column, etc. In Figs. 299 and 300 (0, C), are seen the two 

 masses of cells (protovertebrae) situated by the sides of the neural canal, which 

 are destined to be developed into the vertebras. These cells extend around 

 and encroach upon the chorda dorsalis, and form the bodies of the vertebras. 

 They also extend over the neural canal, closing above, and their processes are 

 called the medullary, or dorsal plates. Sometimes the dorsal plates fail to 

 close at a certain point in the spinal column, and this constitutes the mal- 

 formation known as spina bifida. From the sides of the bodies of the verte- 

 brae, the various processes of these bones are formed. As the spinal column 

 is developed, its lower portion presents a projection beyond the pelvis, which 

 constitutes a temporary caudal appendage, curved toward the abdomen ; but 

 this no longer projects after the bones of the pelvis are fully developed. At 

 the same time the entire vertebral column is curved toward the abdomen, 

 and it is twisted upon its axis, from left to right, so that the anterior face of 

 the pelvis presents a right angle to the upper part of the body ; but as the 

 inferior extremities and the pelvis are developed, the spine becomes straight. 

 The vertebrae make their appearance first in the middle of the dorsal region, 

 from which point they rapidly extend upward and downward, until the sp'inal 

 column is complete. 



At the base of the skull, on either side of the superior prolongation of the 

 chorda dorsalis, are two cartilaginous processes, which are developed into the 

 so-called cranial vertebrae. In this cartilaginous mass, three ossific points 

 appear, one behind the other. 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 developed from the connective tissue, without the inter- 

 vention of pre-existing cartilaginous structure. At the time when the verte- 

 brae are developed, with their laminae and their spinous and transverse pro- 

 cesses, 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 prominences, 

 which are the first traces of the arms and legs, appear on the body of the 

 embryon. These progressively increase in length, the arms appearing near 

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

 ity is divided into three portions, the arm, forearm and hand, for the upper 



