268 Studies of the Development of the Human Skeleton 



from the distribution of the nerves to the posterior limb, the ilium is 

 differentiated in a region anterior to the site of its permanent attachment 

 and the differential activities which it stimulates in the sacral vertebrae 

 are exerted first on the more anterior of these vertebrae. 



The two limbs do not always call forth a similar response on each side 

 of the body. Thus Paterson, 93, found asymmetry of the sacrum in 8.3% 

 of the instances he studied. 



Assuming the specific differentiation in the lumbar, sacral and coccy- 

 geal regions of the spinal column represents a response to stimuli arising 

 in part from the developing limb, we may turn to a consideration of the 

 differentiation thus brought about in each of these regions. Attention 

 will here be directed chiefly to the more salient differences between de- 

 velopment in the distal half of the vertebral column and that recently 

 described for the thoracic region. 



LUMBAR VERTEBRA. 



Eosenberg, 76, seems to have been the first to take up a detailed study 

 of the early development of the lumbar vertebrae in man. He described 

 the costal rudiment of these vertebrae and found in several embryos that 

 this rudiment of the 20th vertebra had given rise to a cartilagenous 

 13th rib. A careful study of a large number of human embryos 

 has led me, however, to the conclusion (1904) that a 13th rib is no 

 more frequent in the embryo than in the adult and that in the series 

 studied by Eosenberg it must have been unusually frequent. Holl, 82, 

 found no 13th rib in the embryos which he examined. He also came to 

 the conclusion that the transverse processes of the lumbar vertebrae do not 

 represent ribs. Most investigators rightly disagree Avith him on this 

 point. 



The development of the external form of the lumbar vertebras in a 

 series of embryos belonging to the Mall collection is shown in Figs. 1-13, 

 Plates I-V. 



The bodies of the lumbar vertebrae during the earlier periods of differ- 

 entiation are essentially like those of the thoracic vertebrae. In embryos 

 over 12 mm. long, however, the former become progressively thinner, 

 broader and longer than the latter. The intervertebral disks and the 

 enveloping ligamentous tissue are similar in both regions. •In the 

 thoracic region the canal of the chorda dorsalis lies nearer the ventral 

 surface of the vertebral column than it does in the lumbo-sacral region. 

 The marked alterations in the curvature of the spinal column which occur 

 during embryonic development seem especially associated with changes 

 in the intervertebral disks. 



