ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 647 



SUMMARY. 

 A. The Vertebral Column. 



1. During development the vertebral column passes through 

 several (from lower to higher) morphological conditions, of which the 

 lower are permanently preserved in the inferior classes of Vertebrates, 

 whereas in the higher classes they appear only at the beginning of 

 development and are then replaced. 



2. In the axial skeleton three different stages of development are 

 distinguished : 



(1) As chorda dorsalis (notochord), 



(2) As cartilaginous and 



(3) As osseous vertebral column. 



3. The chorda is developed out of a tract of cells (chorda-entoblast, 

 fundament of the chorda) lying below the neural tube and belonging 

 to the inner germ-layer, from which it is detached by abstriction 

 (chordal folds). 



4. The chorda is a rod composed of vesiculated cells and bounded 

 superficially by a firm sheath ; it begins with a pointed end beneath 

 the mid-brain vesicle (in the region of the future sella turcica of the 

 cranial floor) and reaches to the blastopore (primitive groove). 



5. The chorda persists as a permanent skeletal structure in 

 Amphioxus and the Cyclostomes. 



6. A cartilaginous vertebral column is found in the adults of the 

 Selachians and some of the Ganoids, while in the remaining Verte- 

 brates it appears more or less during development as a forerunner 

 of the bony vertebral column. 



7. The cartilaginous vertebral column is developed by histological 

 metamorphosis out of embryonic connective tissue, a part of which 

 envelops the chorda as skeletogenous chordal sheath, and a part 

 forms a thin continuous envelope (membranous vertebral arches) 

 around the neural tube. 



8. The process of chondrification begins on both sides of the 

 chorda, progresses around it both above and below, and thus forms 

 a cartilaginous ring, the body of the vertebra, from which the 

 process of chondrification advances dorsnlly into the membranous 

 envelppe of the neural tubes, producing the arches of the vertebrae 

 and ceasing with the formation of the vertebral spines. 



9. It is not until the beginning of the process of chondrification 

 in the unsegmented, connective-tissue, skeletogenous chordal sheath 



