28 



HUMAN ANATOMY. 



interesting to note, that in the connecting Hnk between the vertebrates and 

 invertebrates, the amphioxus, the notochord remains as the permanent and sole 

 spinal axis. 



The history of the notochord in man and mammals presents three stages : (a) 

 it exists as an imbroken cord which extends uninterruptedly through the series 

 of cartilaginous vertebrae ; (b) the notochord suffers segmentation in such manner 

 that the breaks in its continuity correspond to the vertebral bodies, conspicuous 

 proliferation and local increase in its substance, on the contrary, marking the 



Fig. 30. 



Fig. 31. 





Transverse sections through axis of early human embryo of about fifteen days, showing formation of notochord 

 from entoblast. High magnification. {After Kollmann.) n, neural canal; ch, cells forming notochord differenti- 

 ating from entoblast (e) ; m, mesoblast ; s, early somite ; d, sections of primitive aortae. 



position of the intervertebral disks in which the chordal tissue during the first 

 months after birth is represented by a considerable mass of central spongy 

 substance ; (c) atrophy of the remains of the notochord, resulting in the entire 

 disappearance of the chordal tissue within the vertebrae and the reduction of the 

 proliferated intervertebral cell-mass to the pulpy substance existing within the 

 intervertebral disks. 



The cephalic end of the notochord in man corresponds in position to the dorsum 

 sellae, and marks the division of the skull into two parts, that lying in front of 



Fig. 32. 



Paraxial mesoblast 



Ectoblast 



Amniotic cavity 



Somatopleura 



Parietal 

 mesoblast 



Open Ento- Chorda Neural Visceral IJody-cavity 

 gut blast tube mesoblast 



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



Body-cavity Splanchno 

 pleura 



the termination of the notochord, the prechordal portion, and that containing the 

 notochord, the chordal portion ; the latter is sometimes described as the vertebral 

 segment of the skull. 



The CcElom, — The downward growth of the neural ectoblast and the upward 

 extension of the chordal entoblast effect a division of the mesoblast along the 

 embryonic axis into two sheets (Fig. 28). These latter undergo further division 

 in consequence of the formation of a cleft within their substance, as the result of 

 which the mesoblast becomes split into two layers enclosing a space, the ccelom, or 

 primary body-cavity (Fig. 29). 



