170 Development of Tlioracic VertebrsE in Man 



ventral arch, but in the thoracic region of mammals it fuses with the 

 antero-lateral portion of the corresponding vertebral body. After its 

 junction Avith this the pedicle increases in size but otherwise shows no 

 marked alteration of form. 



The transverse process is at first a short projection which lies at some 

 distance from its corresponding rib (Fig. 26), The cartilagenous rib 

 rapidly increases in size and at the same time the transverse process grows 

 outward and forward to meet it (Figs. 29, 32 and 34), At first the 

 developing cartilage of the rib and that of the transverse process are 

 embedded in a continuous blastema, but before chondrification has pro- 

 ceeded far, branches from successive intervertebral arteries become anas- 

 tomosed in the area between the neck of the rib and the transverse 

 process and separation is effected (Figs. 36, 38 B and 39). 



Between the extremity of the transverse process and the rib a joint is 

 developed (Figs. 39, 40, 41 and 42), and the surrounding blastema con- 

 verted into costo-transverse ligaments. 



The articular processes develop slowly from the cartilage. Extension 

 takes place anteriorly, A. A. Pr., and posteriorly, F. A. Pr., in the inter- 

 dorsal membrane. In an embryo of 14 mm. (Figs. 25, 26 and 27) these 

 articular plates are separated by a distinct interval. In one of 17 mm. 

 they have approached each other very closely (Fig. 37) ; and in one of 

 20 mm. not only do the articular processes show distinctly more form 

 (Figs. 28, 29 and 30), but in addition the superior articular process 

 slightly overlaps the inferior (Fig. 38). This overlap of the superior 

 articular processes is distinctly more advanced in an embryo of 28 mm. 

 (Fig, 39), and still more so in one of 33 mm. (Figs, 31-33), In an 

 embryo of 50 mm, (Figs. 34, 35 and 40) conditions essentiall}'' like the 

 adult have been reached. 



The laminar processes scarcely exist in Embryo CXLIV (Fig. 26). 

 In Embryo XXII (Fig. 29) they have begun to project posteriorly to the 

 region of the articular processes (Fig. 29). The dense embryonic con- 

 nective tissue covering the laminar processes at this stage gives attach- 

 ment to a membrane covering, the dorsal musculature, F. D. M., and to 

 a membrane surrounding the spinal cord, M. R. D. This accounts for 

 the two projections seen dorsally on the side of the model representing 

 the membranous tissue. In Embrj^o CXLV, length 33 mm., the laminar 

 processes extend well toward the dorsal line (Figs. 32 and 33) ; in Em- 

 bryo LXXXIV, length 50 mm. (Figs. 34, 35 and 40), they completely 

 encircle the spinal canal and from the region of fusion of each pair a 

 spinous process extends distally, though not so far as in the adult. 



Alterations in the cartilage of the neural processes preliminary to 



