IV.] 



THE HEART. 

 FIG. 29. 



83 



N.C. 



F.So. 



DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE Axis OF 

 AN EMBRYO. 



The section is supposed to be made at a time when the head- 

 fold has commenced but the tail-fold has not yet appeared. 

 N.C. neural canal, closed in front but as yet open behind. Ch. 

 notochord. The section being taken in the middle line, 

 the protovertebrse are of course not shewn. In front of 

 the notochord is seen a mass of uncleft mesoblast, which 

 will eventually form part of the skull. D. the commencing 

 foregut or front part of the alimentary canal. F. So. 

 Somatopleure, raised up in its peripheral portion into the 

 amniotic fold Am. Sp. Splanchnopleure. At Sp. it forms 

 the under wall of the foregut ; at F. Sp. it is turning round 

 and about to run forward. Just at its turning point the 

 cavity of the heart Ht. is being developed in its mesoblast. 

 pp. pleuroperitoneal cavity. A epiblast, B mesoblast, C 

 hypoblast, indicated in the rest of the figure by differences in 

 the shading. At the part where these three lines of reference 

 end the mesoblast is as yet uncleft. 



very little further back, close under the blind end of 

 the foregut, the cleavage (at the stage of which we are 

 speaking) begins, and the somatopleure, F. So, and 

 Splanchnopleure, F. Sp. diverge from each other. They 



62 



