168 Dovelopniont of Thoracic A'ertebra^ in Mnn 



Chondrogenous Period. 



The tissue rehitions during this period have been carefully stndied in 

 representatives of most of the chief groups of vertebrates. The form of 

 the early structures has been less accurately determined because most 

 investigators have avoided the somewhat laborious methods of plastic 

 reconstruction. 



On each side of the blastemal vertebra three primary centers of chondri- 

 fication appear at about the same time, one for neural process, one for 

 the costal process and one for the vertebral body. Fig. 7, Plate II, shows 

 these centers as they appear in a cross section at an early period. Figs. 

 25, 26 and 27, Plate VI, show the early cartilages of an embryo slightly 

 older, CXLIV, length 14 mm., age 5-| weeks. 



The cartilages of the vertebral body develop by a transformation of 

 the tissue lying between the primitive vertebral disks and surrounded by 

 the interdiscal membrane. A considerable part of this tissue is derived 

 from the posterior surface of each primitive disk. At first the cartilage 

 of the left side is separated from that of the right by the perichordal 

 septum. Soon this is broken through and the tAvo anlages of cartilage 

 become united about the chorda. In the thoracic region this imion seems 

 to take place at about the same time dorsally that it does ventrally. A 

 sagittal section of an embryo at this stage is shown in Fig. 18. The 

 chorda dorsalis is surrounded by a perichordal sheath. The latter is 

 encircled by dense intervertebral disks which alternate with light cartil- 

 agenous rings. The latter are surrounded by perichondriimi which is less 

 condensed than the tissue of the disks, but more so than that of the 

 bodies and about the same as that of the perichordal sheath. Ventrally 

 and dorsally a longitudinal ligament has been differentiated from the 

 surrounding mesenchyme. 



It is probable that the disks seen in this section are formed in part 

 from the primitive disks, in part from the posterior layer of the anterior 

 sclerotome halves ; in other words, that each is formed about the rudiment 

 of the fissure of v. Bbner. Compare Figs. 17 and 18. The tissue is con- 

 centrically arranged in a way somewhat resembling that of the interver- 

 tebral disks of the adult. 



The perichordal tissue rapidly decreases in thickness. At the same 

 time the cartilage of the vertebral bodies grows also at the expense of the 

 intervertebral disks (Figs. 19, 20, 21 and 22). According to Schultze, 

 96, the cartilages of the bodies finally fuse to form a continuous car- 

 tilagenous column. This does not seem to be the case in man. In all of 

 the embryos belonging to the collection of Prof. Mall some mem- 

 branous tissue may be seen separating completely the successive bodies, 



