Charles E. Bardeen 167 



Meanwhile the ventral processes of the thoracic vertebra extend well 

 into the thoracic wall, giving rise to primitive ribs, illustrated in Fig. B, 

 Plate II, in the article of Bardeen and Lewis, oi. 



Development proceeds rapidly. In Embryo CIX, length 11 mm., age 

 about five weeks (Figs. 14-16), the conditions are as follows: The neural 

 processes are somewhat better developed than those of the preceding stage, 

 but otherwise are similar in character. The costal processes are consid- 

 erably farther developed (Bardeen and Lewis, oi, Plate V, Fig. E). 

 At the angle between the neural and costal processes opposite where they 

 join the primitive disks a transverse process, but slightly indicated 

 at the preceding stage, is now fairly clearly marked. Each primitive 

 disk has become further hollowed out at its posterior surface, owing, in 

 all probability, to the conversion of its tissue into that of the area 

 between the disks. The interdiscal membrane (Ids. m.), on the other 

 hand, has become thicker and has extended anteriorly and posteriorly 

 about the area between the disks so that this has become completely 

 enclosed (Figs. 14, 15 and 16). The tissue of each segment immedi- 

 ately anterior to the primitive disk has become greatly thickened and the 

 line between it and the disk indistinct. 



The area between each two primitive disks is still divided by the peri- 

 chordal septum (Fig. 7). Each half represents the anlage of a chon- 

 drogenous center of the vertebral body. Formation of cartilage has not, 

 however, begun. The thickening of the ventral margin of the primitive 

 disk at this stage represents the "hypochordal Spange," which Froriep 

 has shown to play an important part in the development of the vertebrae 

 of birds and of the atlas in mammals. It has merely a transitory exist- 

 ence in the thoracic region of man. 



Summary. — To sum up briefly, we may say that during the blastemal 

 period each scleromere becomes divided into two portions, an anterior 

 and a posterior, characterized by a much greater condensation of tissue 

 in the posterior. From this condensed tissue arises a primitive vertebra 

 of Eemak, or scleromere, with dorsal (neural) and ventral (costal) pro- 

 cesses and a disk uniting them to the mesenchyme condensed about the 

 chorda dorsalis. From the tissue of the anterior half of each sclerotome 

 arise membranes which serve to unite the dorsal processes of the sclero- 

 meres, interdorsal memhranes, and to cover in the areas between the suc- 

 cessive disks, interdiscal membranes. The primitive disks become hol- 

 lowed out posteriorly by a loosening up of their tissue and strengthened 

 anteriorly by a condensation of the tissue immediately bounding the fis- 

 sure of V. Ebner. The area between each two disks is bilaterally divided 

 by a membrane springing from the perichordal sheath. The formation 

 of a cartilagenous skeleton now begins. 



