636 



THE HEART. 



tubes (fig. 358), which eventually coalesce into an unpaired 

 structure. 



In Mammals the two tubes out of which the heart is formed appear at 

 the sides of the cephalic plates, opposite the region of the mid- and hind- 

 brain (fig. 358). They arise at a time when the lateral folds which form 

 the ventral wall of the throat are only just becoming visible. Each half of 

 the heart originates in the same way as the whole heart in Elasmobranchii, 

 etc. ; and the layer of the splanchnic mesoblast, which forms the muscular 

 wall for each part (ahh), has at first the form of a half tube open below to 

 the hypoblast. 



On the formation of the lateral folds of the splanchnic walls, the two 

 halves of the heart become carried inwards and downwards, and eventually 



FlG. 359. TWO DIAGRAMMATIC SECTIONS THROUGH THE REGION OF THE 

 HIND-BRAIN OF AN EMBRYO CHICK OF ABOUT 36 HOURS ILLUSTRATING THE 

 FORMATION OF THE HEART. 



fib. hind-brain ; nc. notochord ; E. epiblast ; so. somatopleure ; sp. splanchno- 

 pleure ; d. alimentary tract ; hy. hypoblast ; hs. heart ; of. vitelline veins. 



