THE NOTOCHORD. 



27 



mesoblast. The subsequent differentiation of the walls of the neural tube will be 



more fully considered in connection with the nervous system ; suffice it here to 



state that the cephalic portion expands into the brain vesicles, and subsequently 



becomes the brain with the contained 



ventricles, while the remainder of ^^'^' ^7- 



the tube becomes the spinal cord, 



enclosing the minute central canal. 



The Notochord. — Coinci- 

 dently with the formation of the med- 

 ullary groove the entoblast opposite 

 the bottom of that furrow exhibits 

 proliferation and thickening ; the 

 group of cells thus differentiated be- 

 comes separated from the general 

 mass of the inner layer and takes 

 up a position immediately below the 

 neural tube (Figs. 30, 31). This 

 isolated column of entoblastic cells 

 constitute the notodiord, or chorda 

 dorsalis, the earliest suggestion of 

 the cardinal" vertebrate axis around 

 which the parts of the early embryo 



are symmetrically arranged. While for a time constituting the sole longitudinal axis 

 of the embryo, extending from a point near the cephalic pole, which corresponds later 



Medullary fold 

 — — Medullary groove 



Wall of blastoder- 

 mic vesicle 



Primitive streak 



Embryonic area 



Embryonic area of rabbit of about eight and one-half days, 

 seen from the surface. X 24. (KoUmatin.) 



Medullary fold 



Fig. 28. 



Groove 



Medullary fold 



Transverse section of rabbit embryo of about eight and one-half days. X 80. Future neural canal is represented by 



widely open groove. 



Ectoblast 

 Mesoblast 



Body-cavity 



Amniotic sac 



Fig. 29. 



Closing neural canal 



Somatopleura 

 Body-cavity 



Visceral mesoblast Entoblast Chorda Open gut-tube 'Splanchnopleura 



Transverse section of rabbit embryo of about nine and one-quarter days. X 80. Neural canal is just closing. 



to the base of the skull, to the caudal extremity, the notochord is but a temporary 

 structure, and subsequently is supplanted by the true vertebral column. It is 



