EMBRYOLOGY 





of the tube forms the medulla spinalis or spinal cord; from its ectodermal wall 

 the nervous and neuroglial elements of the medulla spinalis are developed while 

 the cavity persists as the central canal. 



THE NOTOCHORD. 



The notochord (Fig. 19) consists of a rod of cells situated on the ventral aspect 

 of the neural tube; it constitutes the foundation of the axial skeleton, since around 

 it the segments of the vertebral column are formed. Its appearance synchronizes 

 with that of the neural tube. On the ventral aspect of the neural groove an axial 

 thickening of the entoderm takes place; this thickening assumes the appearance 

 of a furrow the chordal furrow the margins of which come into contact, and so 

 convert it into a solid rod of cells the notochord which is then separated from 

 the entoderm. It extends throughout the entire length of the future vertebral 



Ectoderm ,, 



Neural canal Primitive Wolffian 



/ segment duct Celom 



Somatic mesoderm 



Entoderm '' 



Nctochord 



Aorta Splanchnic mesoderm 



FIG. 19. Transverse section of a chick embryo of forty-five hours' incubation. (Balfour.) 



column, and reaches as far as the anterior end of the mid-brain, where it ends in 

 a hook-like extremity in the region of the future dorsum sellse of the sphenoid 

 bone. It lies at first between the neural tube and the entoderm of the yolk-sac, 

 but soon becomes separated from them by the mesoderm, which grows medial- 

 ward and surrounds it. From the mesoderm 

 surrounding the neural tube and notochord, 

 the skull and vertebral column, and the 

 membranes of the brain and medulla spinalis 

 are developed. 



THE PRIMITIVE SEGMENTS. 



Toward the end of the second week 

 transverse segmentation of the paraxial 

 mesoderm begins, and it is converted into 

 a series of well-defined, more or less cubical 

 masses, the primitive segments (Figs. 18, 

 19, 20), which occupy the entire length of 

 the trunk on either side of the middle line 

 from the occipital region of the head. Each 

 segment contains a central cavity myocoel 

 which, however, is soon filled with angular 

 and spindle-shaped cells. 



The? primitive segments lie immediately 

 under the ectoderm on the lateral aspect of 

 the neural tube and notochord, and are con- 

 nected to the lateral mesoderm by the inter- 

 mediate cell-mass. Those of the trunk may 



Yolk-sac 



Cut edge of amnion 



Primitive segments 



Neural folds 



Neurenteric canal 



Fia. 20. Dorsum of human embryo, 2.11 mm. in 

 length. (After Eternod.) 



