DEVELOPMENT OF THE SKELETON, MUSCULAR SYSTEM, AND SKIN. 915 



human subject, at from the ninth to the twelfth year, 

 just described are represented in Fig. 297. 



The processes of development 



Vertebral Column, etc. In Figs. 295 and 296 (c, c), are seen the two masses of cells, 

 situated by the sides of the neural canal, which are destined to be developed into the 

 vertebrae. These cells extend around and encroach upon the chorda dorsalis and form 



the bodies of the vertebrae. They also ex- 

 tend over the neural canal, closing above, 

 and these processes are called the medul- 

 lary, 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 vertebrae, the va- 

 rious 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 



FIG. 297. The first six cervical vertebrae of the embryon 

 of a rabbit one inch in length. (Robin.) 



rt, b, cephalic portion of the notocorde exposed by the re- 

 moval of the cartilage ; b, portion of the chorda dor- 

 salis slightly enlarged, which, in this embryon, was 

 situated between the atlas and the occipital bone ; c, 

 odontoid process ; d, base of the odontoid process ; e, 

 inferior, or second part of the body of the axis; /, fc, 

 enlargements of the chorda dorsalis between the ver- 

 tebrae ; a, cartilage of the lateral portion of the atlas ; 

 h, lateral portion of the axis ; i, i, transverse apophy- 

 ses of vertebrae. 



FIG. 298. Human embryo, about one month old, show- 

 in^ the large sine of the head and upper parts of fie 

 body, the twisted form of the spinal column, the 

 rudimentary condition of the upper and lou-er 

 extremities, and the rudimentary tail at the end 

 of the spinal column. (Dalton.) 



upper part of the body ; but, as the inferior extremities and the pelvis become 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 spinal column is complete. 



At the base of the skull, on either side of the superior prolongation of the chorda dor- 

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

 tebra. 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 poste- 

 rior 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 pre- 



