Charles E. Bardeen 269 



The neiiro-cosial processes of the lumbar vertebra are also at first 

 similar in form to those of the thoracic vertebrae. This is the case in 

 Embryo II, length 7 mm.. Kg. 1, Plate I ; CLXIII, length 9 mm., Fig. . 

 2, Plate II; and CIX, le^igth 11 mm., Figs. 3 and 4, Plate II. 



Marked differences in the costal processes are to be seen when chondro- 

 fication begins. Thns while the costal process of the 12th thoracic ver- 

 tebra has early a separate center of chondrofication (Embryo CLXXV, 

 length IS mm., Fig. 14, Plate VI and Embryo CXLIV, length IJf mm.. 

 Fig. 5, Plate III), the processes of the lumbar vertebrae remain for a 

 considerable period dense masses of mesenchyme (Embryo CLXXV, 

 length 13 mm.. Figs. 15 and 16, Plate A^I, and Embryo CCXVI, length 

 11 mm.. Figs. 18 and 19, Plate VI). Finally, however, they undergo 

 chondrofication at the base (Embryo XXII, length 20 mm., Figs. 21 and 

 22). I have been unable to determine whether this chondrofication 

 always takes place from a separate center, as it certainly often does, or 

 sometimes represents merely an extension into the costal mesenchyme of 

 cartilage from the -transverse process. I incline to the former view. 



Sometimes the costal element of the 1st lumbar vertebra may remain 

 for a considerable period separate from the cartilage of the transverse 

 process. This is true of the right side (left in the figure) of Embryo 

 XLV, length 28 mm.. Fig. 24. But usually at an early period the costal 

 and transverse processes become intimately fused (Embryo XXII, length 

 20 mm.. Fig. 21; XLV, length 28 mm., Fig. 24, right side of figure; 

 and Fig. 25 ; Embryo LXXXIV, length 50 mm.. Figs. 27 and 28) . The 

 " transverse " process of the adult lumbar vertebra represents in the main 

 an ossification of a membranous, not cartilagenous, extension of the fused 

 costal element (C. Pr., Fig. 28). 



At first the neural processes of the lumbar vertebrae are essentially 

 like those of the thoracic (Embryo CXLIV, length IJ/. mm.. Fig. 5, 

 Plate III). Union of the pedicles with the cartilage of the body takes 

 place and the laminae extend out dorsally in a similar manner in each. 

 It is in the transverse and articular processes that the chief character- 

 istic differentiation takes place. 



The lumbar transverse processes are broader and much shorter than 

 the thoracic. At an early period, as mentioned above, they become inti- 

 mately united to the costal processes. In the dense mesenchyme between 

 the region of the transverse process and the costal element there is com- 

 monly developed no loose vascular area such as serves to separate the 

 neck of the developing rib from the transverse process in the thoracic 

 region. The occasional appearance of a foramen in a transverse process 

 of a lumbar vertebra has led to the supposition that they may occur 



