146 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



into its posterior end the neurenteric canal opens. Afterwards it 

 becomes a solid rod composed of cells of a peculiar type. A 

 sheath is formed round the notochord by cells of the paraxial 

 mesoblast (Fig. 116), which grow inwards and surround it. 



Epiblast. 



myotome. 



Wolffian duct. 

 Mullerian duct. 

 Wolffian ridge 

 Somatopleure 



neural canal 



sclerotome 



notochord 

 intermediate cell mass 

 visceral mesoblast 



gut. 



Fig. 116. — Diagrammatic transverse section of a human embryo at the end of the 



3rd week. 



These cells form the sclerotome and spring from the inner parts 

 of the primitive segments (Fig. 116). At the same time the 

 cells of the sclerotome also surround the neural tube. From 

 these cells which grow inwards and surround the notochord and 

 neural canal, the spinal column is formed and also the basi- 

 occipital and part of the basi-sphenoid bones of the skull (Fig. 

 117). 



What becomes of the Notochord. — In the second month of 

 foetal life the notochord begins to disappear ; the bodies of the 

 vertebrae and parachordal cartilages form in its sheath and 

 constrict it. The parachordal cartilages form the basi-occipital 

 and basi-sphenoid — the basal part of the skull — behind the i 

 pituitary fossa. As they form, the notochord is obliterated \ 

 between them. Eternod, however, found the anterior part of 

 the notochord on the dorsal wall of the pharynx in the human 

 embryo, so that the parachordal cartilages are evidently de- 

 veloped on its dorsal aspect. The odontoid process represents 

 the body of the atlas and the suspensory ligament the disc 

 between the occipital bone and atlas. A remnant of the noto- 



