VERTEBRAE 



251 



looser mesenchyme, which is derived, like the tissue intervening between the 

 neural processes, from the anterior portions of the sclero tomes. The plates 

 are at first relatively thick, but the tissue forming them becomes loosened 

 posteriorly, and is added to the loose investment of the notochordal sheath, 

 while in front it is condensed where it adjoins the fissure of Ebner. This 

 thickened band is the rudiment of the permanent disc, and the looser tissue 

 intervening between adjoining discs is converted into the body of the 

 permanent vertebra. It follows from this description that the future body 

 is contributed to by the anterior portions of a sclerotome pair, and also by 

 the posterior portions (primitive plates) of the preceding pair, so that a new 

 segmentation of the skeletal axis is effected which alternates with the primary 

 myotomic segmentation. The formation of the vertebral bodr is brought about 



notoch. disc 



neural process 



__ intersegmental 

 artery 



i-ostaLprocess 



costal process 



neural process 



inlet-dorsal-, 

 membrane 



notochord peric/iordal septum 



FIG. 805. VIEWS OF MODELS OF BLASTEMAL (MEMBRANOUS) STAGE OF VERTEBRAL COLUMN : 

 A. FROM AN EMBRYO OF 7 MM., VENTRAL ASPECT, x 33 diameters. B. FROM AN EMBRYO 

 OF 9 MM., DORSAL ASPECT, x 25 diameters. C. FROM AN EMBRYO OF 11 MM., LATERAL 

 ASPECT, x 25 diameters. (After Bardeen.) 



as follows : the notochordal sheath becomes prolonged dorso-ventrally into a 

 kind of septum (fig. 305, C), which extends between the primitive'plates and separates 

 the loose mesenchyme, alluded to above, into a right and left moiety ; at the same 

 time the superficial layers of the intervening tissue become condensed into a 

 continuous lamella uniting the plates, and enclosing the looser tissue on each side 

 of the septum. This enclosed tissue now becomes converted into cartilage. 

 There are necessarily at first two chondrogenetic centres, but soon the septum 

 becomes implicated,|and the notochord is enclosed in a continuous cartilaginous 

 ring. According to 0. Schultze, the cartilage formation extends also through the 

 primitive intervertebral plates, so that the column becomes for a time a continuous* 

 rod of cartilage, and the permanent discs are formed in this secondarily by the 

 conversion of the hyaline into fibro- cartilage, the only persistent portion of the 

 primitive membranous plates being the annulus fibrosus of the intervertebra 



