Development of the Second Day 143 



THE HEART 



During the first half of this day the heart 

 makes its appearance as a sort of hollow- 

 ing out of the mesoblast in the anterior end 

 of the embryo, under the fore-gut (Fig. 48, 

 Endd). The walls of the heart consist of an 

 outer muscular coat, formed of the splanchnic 

 mesoblast, and an endothelial lining whose 

 origin is said, by some, to be from the 

 entoblast. 



The heart consists, at first, of two longi- 

 tudinal vessels, in contact anteriorly but di- 

 verging posteriorly, which together form a 

 V-shaped structure, with the point of the V 

 towards the head of the embryo. 



As development proceeds, the arms of the 

 V fuse together, from before back, thus con- 

 verting the V into a Y, with the stem of the 

 Y towards the head. The cavities of the two 

 tubes that thus build up the heart are at first 

 quite distinct, even after the two tubes have 

 come in contact with each other : but they 

 soon unite to form one cavity, the endothelial 

 lining remaining as two distinct cavities for 

 a short time after the muscular walls have 

 fused. The muscular walls of the tubes are 

 incomplete on the dorsal side, for a time, 



