Charles E. Bardeen 169 



but in embryos between 20 and 40 mm. in length tliis membrane in tlie 

 vicinity of the chorda dorsalis is very thin. At the periphery of the 

 disks the annnlns fibrosns is meanwhile differentiated more and more 

 into a condition resembling the adnlt (Figs. 17-23, Plate V). 



The chorda dorsalis at the period shown in Fig. 18 is of about the 

 same size at the level of the disks and between them, but as the bodies 

 increase in size at the expense of the disks the chordal canal becomes en- 

 larged in the intervertebral areas and constricted at the centers of the 

 bodies (Figs. 19, 20 and 21). The chorda loses its continuity and the 

 chordal cells become clumped in the vicinity of the disks (Figs. 21 and 22) 

 and finally spread out there in the form of a flat disk (Fig. 23) . At this 

 last period the perichondrium of the bodies is again becoming well 

 marked and the portion of each intervertebral disk in the vicinity of the 

 chorda dorsalis is better developed than during the stages immediately 

 preceding. The chordal canal long remains in the vertebral body (Figs. 

 23 and 24). 



The cartilage of the bodies in Embryo CXLIV(Fig. 18) is of an early 

 embryonic hyaline type. At a slightly later stage (Fig. 19) two regions 

 may be distinguished, a central and a peripheral. The central cartilage 

 is denser than that of the preceding stage, while the peripheral cartilage 

 resembles it. Gradually the cartilage at the center of the body undergoes 

 further changes. The cells enlarge and become sharply set off against the 

 intercellular substance (Figs. 22, 23 and 24), and finally an invasion of 

 blood vessels takes place, chiefly from the posterior surface (Fig. 23). 

 These changes in the cartilage, represented also in Fig. 41, Plate VII, 

 are preliminary to ossification. 



Deposit of calcium salts and actual ossification begins in the distal 

 thoracic and proximal lumbar vertebrse of embryos about 5 to 7 cm. long 

 and three months of age. Fig. 42 shows a center of ossification in an 

 embryo of 70 mm. 



During the development of the vertebral bodies changes have been 

 active in the neural cartilages. At the period represented in Fig. 7, Plate 

 II, the neural cartilage is a small, flat body situated in the dorsal process 

 of the scleromere; from this as a center, pedicular, transverse, anterior 

 (superior) and posterior (inferior) articular, and laminar processes are 

 rapidly developed. This structural differentiation is best followed in the 

 figures representing the models (Figs. 25-36). The pedicular processes 

 are at first slender rods (Fig. 26), each of which grows out towards and 

 finally fuses with its corresponding vertebral body. Froriep has shown 

 (83) that in the chick this process forms a more essential element of the 

 body than in mammals. In the atlas it forms a lateral half of the 



