CHAPTER XXXII. 



THE FIRST HALF OF THE SECOND DAY 

 (Twenty-four to Thirty-six Hours) 



It is well at this point to continue with the vascular system, thus 

 giving a connected account of how the heart and the various blood ves- 

 sels are formed. 



The paired primordia of the heart, already mentioned, grow mesiad 

 and fuse, to form a thin-walled tube which becomes the endothelial lining 

 of the heart (Fig. 275). The muscular walls of the heart are formed by 

 the addition of an external layer of mesoderm. This is understood the 

 better by noting that the splanchnic mesoderm on each side forms a fold 

 around the endothelial rudiment and fuses both dorsally and ventrally 



Fig. 275. 



Cross section of A, through head of 2 day chick embryo in the region 

 of the mid brain. B, through posterior region of head at the end of 3 days. 

 ao, aorta ; and, otic anlage ; c, heart anlage ; ch, notochord ; d, fore-gut ; 

 ec., endocardium ; ect, ectoderm ; ent, entoderm ; g.pl., neural crest ; h.h., 

 hindbrain ; m.c., myocardium; m.n., midbrain. (After Marshall.) 



in the midline. For a very short period this fusion remains on the dor- 

 sal aspect, being called the dorsal mesocardium. The ventral fusion 

 forms the ventral mesocardium. The ventral mesocardium breaks away 

 almost immediately, the dorsal mesocardium remaining for a longer time, 

 but then it also' disappears with the exception of the portion at the an- 

 terior and posterior extremities of the heart. The heart is now a short 

 median tube made up of endothelial and rudimentary muscular layers, 

 suspended in a cavity, later to be called the pericardial cavity. Ante- 

 riorly, the heart-tube is continuous with a short pair of vessels extending 

 into the head-fold the ventral aortae already mentioned. Posteriorly 



