Earh' Development of the Cervical ^'erteljrte 



185 



developed merely in connection with the atlas. It Ijecomes cartilaginous 

 later than the neural arch. There are indications of two hilaterally 

 placed centers of chondrification, but fusion with one another and with 

 the neural arches takes place as soon as chondrification is well under 

 way. 



Specific mention must be made of the mode of development of the 

 epistropheus, of the atlas, and, in connection with the latter, of the 

 base of the occipital. 



Epistropheus. — The general mode of development of the epistropheus 

 is like that of the other cervical vertebrae. Its marked distinction comes 

 from its union with the body of the first cervical vertebra. This union 



Basioccipitai 



Hypochordal brace 



Fig. 3. — Sagittal section tlu'oiigb the lateral part of the cervical region of 

 the spinal column of an embryo 14 mm. long. 



takes place through the transformation of the intervertebral disc into 

 cartilage, first lateral to the mid-sagittal plane and later in this plane. 



Atlas. — The base of each neural hemi-arch of the atlas becomes 

 temporarily fused with the body (14 mm. embryo), but this fusion is 

 incomplete and soon is followed by the development of dense fibrous 

 tissue between the arch and the body (Fig. 3). At the same time the 

 hypochordal brace becomes cartilaginous and unites the arches of the 

 atlas in front of the Body. Each costal process becomes fused medially 

 to the basal process, laterally to the transverse process of the corre- 

 sponding hemi-arch. 



