Charles R. Bardeeu 271 



Soon after the stage shown in Figs. 3 and 4 the iliac blastema 

 approaches more closely the vertebral column, usually in the region 

 opposite the costal processes of the 25th and 26th vertebrse. These, then, 

 are stimulated to more active growth and extend in their turn out toward 

 the ilium. The costal processes of the 27th, 28th and 29th vertebrse 

 are likewise stimulated into more active growth. Lateral to the ventral 

 branches of the spinal nerves the tissue derived from the costal elements 

 of these five vertebra3 becomes fused into a continuous mass of condensed 

 tissue (Fig. 37, Plate IX; and Figs. 5 and 6, Plate III). Against the 

 anterior and better developed portion of this the iliac blastema comes to 

 rest (Figs. 5 and 37). From the time of the fusion of the costal ele- 

 ments of the sacral vertebra into a continuous lateral mass of tissue these 

 vertebrge may be distinguished from the lumbar and coccygeal. A^ari- 

 ation in the vertebrae entering into the sacrum occurs in the embryo as in 

 the adult (Bardeen, 04). 



At the period when the iliac blastema comes into contact with the 

 costal mass of the sacrum centers of chondrofication have appeared in 

 the bodies of the sacral vertebras. The bodies, compared with the inter- 

 vertebral disks, are progressively smaller from the first to the fifth 

 CEmbryo CXLIY, Jenf]tli 14 mm.. Fig. 6). Otherwise they present no 

 characteristics of special note. In older embryos this difference becomes 

 less and less marked. 



The neuro-costal processes present features of more specific interest. 

 In Embryo CXLIV, length IJf mm., centers of chondrofication may be 

 observed in the neural processes of the first two sacral vertebras. They 

 are not yet united to the bodies of the vertebrae and are simple in form. 

 Xo cartilage has as yet appeared in the neural processes of the other 

 vertebra. In somewhat older embryos, CCXVI, length 17 mm.. Fig. 38, 

 and CLXXXVIII, length 17 mm.. Fig. 39, the neural processes of all 

 the sacral vertebrae have become chondrofied and distinct. Separate 

 centers of chondrofication may be seen in each of the costal elements. 

 At a slightly later stage, Embryo XXII, length 20 mm., Eig. 10, Plate 

 IV, the extremities of the costal elements of the first three sacral vertebrae 

 have fused with one another and have thus given rise to a cartilaginous 

 auricular surface, and each has fused with the neural arch of the vertebra 

 to which it belongs. The costal elements of the 4th and 5th sacral ver- 

 tebrae are likewise fused to their corresponding neural arches and all of 

 the sacral neural arches are fused to their respective vertebrae. 



Cross-sections illustrate well the relations of the neural and costal pro- 

 cesses to the vertebrae. Embryo CIX, length 11 mm.. Fig. 29, Plate 

 VIII, shows an early blastemal stage in which the sacral vertebras resem- 



