THE HEART. 299 



muscular coat (Fig. 120, RM), formed by the splanchnopleuric 

 layer of mesoblast ; and an inner endothelial lining, RE, con- 

 cerning the origin of which it is difficult to speak with certainty, 

 but which appears, like that of the other blood-vessels, to be 

 derived from the hypoblast. 



The muscular wall (Fig. 120, RM) is at first incomplete dor- 

 sally, but, after the two halves of the heart have united, the mus- 

 cular walls grow in towards the median plane, above the heart, 



MS 



FIG. 127. The anterior end of a Chick Embryo at the thirty-sixth hour of in- 

 cubation, removed from the yolk-sac, and seen from the ventral surface. 

 (Compare Figs. Ill and 112 for other views of an embryo of the same age.) 

 x30. 



Al, first aortic arch, in the mandibular arch. BF, fore-brain. 'BO, optic vesicle. 



jiortion of heart. V V, vitelliue vein, cut short. 



and coalesce so as to complete its w r all. The endothelial tubes 

 of the two halves of the heart remain distinct, though closely 

 apposed, for some time after the muscular walls have coalesced, 

 but ultimately they, also, become continuous. Between the 

 muscular and endothelial walls there is at first a considerable 

 space, filled with a mucous substance (Fig. 120). 



The heart thus forms, about the thirtieth hour, a short and 



