812 



DEVELOPMENT. 



[CH. LVIII. 



Head and Tail Folds. Body-cavity. Every vertebrate animal 

 consists essentially of a longitudinal axis (vertebral column) with 

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

 alimentary canal) beneath. 



Fig. 627. 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 (3Ic], A, epiblast ; C, hypoblast, consisting of 

 a single layer of flattened cells ; Me, medullary canal ; Pu , protovertebra ; Wd, Wolffian 

 duct; So, somatopleur ; Sp, splancbnopleur ; pp, pleuro-periloneal cavity ; ch, noto- 

 chord ; no, dorsal -aorta, containing blood-cells ; v, blood-vessels of the yolk-sac. 

 (Foster and Balfour.) 



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-cavity is developed. In the earliest stages the embryo 

 lies flat on the surface of the yolk, and is not clearly marked off 



N.C 



F.Sb 



Am 



Fig. 628. 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 somato- 

 pleur ; FSp, fold of the splanchnopleur ; the line of reference, Fso, lies outside the 

 embryo in the " moat," which marks off the overhanging head from the amnion ; I), 

 inside the embryo, is that part which is to become the fore-gut ; Fso and Fsp, are both 

 parts of the head-fold, and travel to the left of the figure as development proceeds ; 

 pp, space between somatopleur and splanchnopleur, pleuro-peritoneal cavity ; Am, 

 commencing head-fold of amnion ; JVC, neural canal ; Ch, notochord ; Ht, heart ; 

 A, B, C, epiblast, mesoblast, hypoblast. (Foster and Balfour.) 



from the rest of the blastoderm ; but gradually the head-fold, a 

 crescentic depression (with its concavity backwards), is formed in 

 the blastoderm, limiting the head of the embryo; the blastoderm 

 is tucked in under the head, which thus comes to project 

 above the general surface of the membrane : a similar tucking in 



