374 Studies of the Development of the Human Skeleton 



plea for the presence of 5 vertebrae in the adult coccyx, the more com- 

 monly accepted number of four seems to be a truer estimate of the defi- 

 nite number of bones usually found present. 



The bend of the coccyx which takes place during the third month 

 is an interesting phenomenon. It seems to be associated with the 

 development of pelvic structures. 



SUMMARY. 



In the earlier stages of development the lumbar, sacral and coccygeal 

 vertebrae resemble the thoracic. The blastemal vertebrae arise each from 

 the contiguous halves of two original segments of the axial mesenchyme. 

 Each vertebra exhibits a body from which neural and costal processes 

 arise. The neural processes are connected by " interdorsal " mem- 

 branes. 



As the blastemal vertebrse become converted into cartilage specific 

 diiferentiation becomes more and more manifest. The cartilagenous 

 vertebral bodies and the intervertebral disks are all formed in a similar 

 manner and except for size manifest comparatively slight differences in 

 form. The more distal coccygeal vertebrae are, however, irregular. But 

 the chief specific differentiation is seen in the costal and neural processes. 



In the blastemal neural processes of the thoracic vertebras cartilagenous 

 plates arise from which spring pediclar, transverse, articular and laminar 

 processes. 



In the lumbar vertebrae similar processes arise from the neural carti- 

 lages. The pediclar processes resemble the thoracic but are thicker; 

 the transverse processes are shorter, much thicker at the base and remain 

 bound up with the costal processes; the superior articular processes 

 develop in such a way as to enfold the inferior ; the laminar processes are 

 broad, grow more directly backward than do the thoracic, and on meeting 

 their fellows in the mid-dorsal line fuse and give rise to the typical 

 lumbar spines. The mammillary and accessory processes are developed 

 in connection with the dorsal musculature. 



In the sacral vertebrae the neural cartilages give rise to very thick 

 pediclar processes; to articular processes the most anterior of which 

 develop like the lumbar, while the others long maintain embryonic char- 

 acteristics; to transverse processes which in development are bound up 

 with the costal processes; and to laminar processes which are very slow 

 to develop and of which the last fail to extend far beyond the articular 

 processes. 



In the coccygeal vertebrae the neural processes of the first, and rarely 



