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HANDBOOK OF PHYSIOLOGY. 



ments, which are the surface appearance of cubes of mesoblast, are the 

 mesoblastic somites or protovertebrae. The first three or four of 

 the protovertebrae make their appearance in the cervical region, while 

 one or two more are formed in front of this point: and the series is 

 continued backward till the whole medullary canal is flanked by them 



Fig. 468. Transverse section through dorsal region of embryo chick (45 hrs.). One half of the 

 section is represented ; if completed it would extend as far to the left as to the right of the line 

 of the medullary canal (3fc). A, epiblast; C, hypoblast, consisting of a single layer of flattened 

 cells; Me, medullary canal ; Pv, protovertebra ; Wd, Wolffian duct; So, somatopleure ; -Sp, 

 splanchnopleure; m>, pleuro-peritoneal cavity; ch, notochord; ao, dorsal aorta, containing 

 blood cells ; v, blood-vessels or the yolk-sac. (Foster and Balf our. ) 



(fig. 467). That which is first formed corresponds to the second cervi- 

 cal vertebra. From these somites the vertebrae and the trunk muscles 

 are derived. 



Head and Tail Folds. Body Cavity. Every vertebrate animal con- 

 sists essentially of a longitudinal axis (vertebral column) with a neural 

 canal above it, and a body-cavity (containing the alimentary canal) 

 beneath. 



We have seen how the earliest rudiments of the central axis and the 

 neural canal are formed ; we must now consider how the general body- 



Fig. 469. Diagrammatic longitudinal section through the axis of an embryo. The head-fold 

 has commenced, but the tail-fold has not yet appeared. FSo, fold of the somatopleure ; Fsp, 

 fold of the splanchnopleure; the line of reference, Fso, lies outside the embryo in the "moat," 

 which marks off the overhanging head from the amnion ; Z>, inside the embryo, is that part 

 which is to become the fore-gut ; Fso and Fsp, are both parts of the head-fold, arid travel to the 

 left of the figure as development proceeds ; pp, space between somatopleure and "s'planchnopleure, 

 pleuro-peritoneal cavity; Am, commencing head-fold of amnion; NC. neural canal; Ch. noto- 

 chord ; Ht , heart ; A, B, C, epiblast, mesoblast, hypoblast. (Foster and Balf our. ) 



cavity is developed. In the earliest stages the embryo lies flat on the 

 surface of the yolk, and is not clearly marked off from the rest of the 

 blastoderm : but gradually the head-fold or crescentic depression (with 



