370 Studies of the Development of the Human Skeleton 



regularly in the embryo (Szawlowski, 02, Dwight, 02). The differences 

 between the transverse processes of the 12th thoracic and the first two 

 •lumbar vertebras in several embryos are shown in Plates VI and VII, 

 Figs. 14 to 28. 



The articular processes in the lumbar, as in the thoracic, region are 

 at first flat plates connected by membranous tissue," Fig. 5, Plate III. 

 But in the lumbar region the superior articular process develops faster 

 than the inferior so that each superior process comes partly to enfold the 

 inferior process of the vertebra next anterior. These conditions may be 

 readily followed in Figs. 9, 12 and 13, Plates IV and V, and in Figs. 

 20-28, Plates VI and VII. The mammillary and accessory processes of 

 the adult lumbar vertebra probably represent an ossification of muscle 

 tendons attached to the transverse and articular processes. 



During the development of the vertebrae in embryos from 30 to 50 mm. 

 in length alterations preliminary to ossification, similar to those in the 

 thoracic, occur in the lumbar vertebra. ISTo actual ossification occurs in 

 any of the centers in the latter in embryos less than 5 cm. long in Prof. 

 Mall's collection. It begins in the bodies of the more anterior of the 

 lumbar vertebrae in embryos between 5 and 7 cm. long, and in those 8 cm. 

 long it has usually extended to the more distal. Meanwhile, ossification 

 of the neural processes has extended from the thoracic into the lumbar 

 region and soon may be seen throughout the latter. See Bade, 00. 



SACRAL VERTEBRA. 



Although much has been written about the ossification of the sacral 

 vertebra comparatively little attention has been devoted to their early 

 differentiation. Eosenberg, 76, contributed several important facts, ' 

 although the general conclusions which he drew concerning the trans- 

 formation of lumbar into sacral, and sacral into coccygeal vertebrEe are 

 unw^arranted. Holl, 82, studied more especially the relation of the ilium 

 to the sacrum in adults and embryos and added materially to the knowl- 

 edge of the sacro-iliac articulation. Petersen, 93, has given an incom- 

 plete description of the sacrum in several early human embryos, and 

 Hagen, 00, of one 17 mm. long. 



Figs. 1 to 13, Plates I to V, show the general external form of the 

 sacral vertebrae in embryos of the Mall collection. In Embryo II, length 

 7 mm.. Fig. 1; CLXIII, length 9 mm.. Fig. 2; and CIX, length 11 mm., 

 Figs. 3 and 4, the sacral appear to resemble the lumbar in all essential 

 particulars, although there is a progressive decrease in size from the mid- 

 lumbar region distally. No line of demarcation can be drawn in these 

 embryos between the sacral and lumbar vcrtebrse on the one side and tlie 

 sacral and coccygeal on the other. 



