

81 



by a continuous sheath of mesoderm, which is termed the membranous vertebral 

 column. In this mesoderm the original segments are still distinguishable, but each 

 is now differentiated into 

 two portions, an anterior, 

 consisting of loosely arranged 

 cells, and a posterior, of 

 more condensed tissue (Fig. 

 65, A and J3) . Between the 

 two portions the rudiment 

 of the intervertebral fibre- 

 cartilage is laid down (Fig. 

 (55, C). Cells from the pos- 

 terior mass grow into the 

 intervals between the myc- 

 tomes (Fig. 65, B and C) of 

 the corresponding and suc- 

 ceeding segments, and extend 

 both dorsally and ventrally; 

 the dorsal extensions sur- 

 round the neural tube and 

 represent the future verte- 

 bral arch, while the ventral 

 extend into the body-wall 

 as the costal processes. The 

 hinder part of the posterior 

 mass joins the anterior mass 



,1 j. FIG. 64. Transverse section of a human embryo of the third week 



OI the Succeeding Segment to show the differentiation of the primitive segment. (Kollmann.) CM. 



Aorta, m.p. ^Juscle-plate. n.c. Neural canal, sc. Sclerotome. s p. 

 cutis-plate. 



to form the vertebral body. 



Each vertebral body is there- 

 fore a composite of two segments, being formed from the posterior portion of 

 one segment and the anterior part of that immediately behind it. The vertebral 



Myotome 



| Anterior portion of sclerotome 

 Notochord 



Posterior portion of sclerotome 

 Intermyotomic septum 

 Costal process 



Intervertebral 

 fibrocartilage 



Notochord 





FIQ. 65. Scheme showing the manner in which each vertebral centrum is developed from portions of two adjacent 



segments. 



and costal arches are derivatives of the posterior part of the segment in front 

 of the intersegmental septum with which they are associated. 

 6 



