272 Studies of the Development of the Human Skeleton 



ble the thoracic. Figs. 30, 31 and 32 represent cross-sections through 

 tlie 1st, 2d and 3d sacral vertebrae of Embryo X, length 20 mm. The 

 neural and costal processes in this embryo show cartilagenous centers, 

 but these are fused neither with one another nor with the vertebral body. 

 Embryo CCXXVI, length 25 mm., Figs. 33 and 34, shows fusion of 

 neural and costal processes with the body. The costal processes have by 

 this period given rise to a continuous lateral cartilagenous mass, a portion 

 of which is represented in Fig. 40. This shows an oblique section through 

 the 3d and 4th sacral vertebrse of Embryo LXXXVI, length 30 mm. 

 This cartilagenous lateral mass may likewise be seen in the sacra of 

 Embryo CXLV, length S3 mm.. Figs. 11 and 12, and LXXXIV, length 

 50 mm.. Fig. 13. 



From the primitive neural cartilages there develop pediclar, trans- 

 verse, articular and laminar processes. The pediclar and transverse 

 processes become intimately fused with the costal element as described 

 above. In none of the embryos examined was there seen a separation of 

 costal from transverse process marked by blood-vessels, such as one might 

 perhaps expect to find because of the occasional appearance in the adult 

 of transverse foramina in the lateral processes of the sacral vertebrae 

 (Szawlowski, 02), 



The articular processes in the older embryos retain a more primitive 

 condition than do those of the lumbar and thoracic regions. Figs. 33, 

 34, 35 and 36 show cross-sections through the articulations of the neural 

 processes. 



The laminar processes of each side are still separated by a considerable 

 interval in embryos of 50 mm., although at this period the lumbar region 

 is nearly enclosed. Compare Figs. 28 and 36. 



Changes in the cartilages preliminary to ossification occur both in the 

 bodies and in the neural processes of the sacral vertebras at a period 

 quickly following their appearance in the lumbar region. Thus iji 

 Embryo LXXlX, length 33 mm., changes of this nature may be followed 

 as far as the 5th sacral vertebra. It is well known, however, that actual 

 ossification in the more distal sacral vertebrae takes place considerably 

 later than in the thoracico-lumbar region. The primary centers of ossi- 

 fication correspond with the centers of chondrofication except that there 

 is a single center in place of two centers for each body, and one center 

 instead of two for each neuro-costal processes of the two more distal 

 sacral vertebrae. Posth, 97, has recently contributed a valuable paper on 

 the subject of sacral ossification. 



