FIRST HALF OF SECOND DAY 



471 



the heart-tube is continuous with the vitelline or omphalo mesenteric 

 veins (Fig. 276). 



The two vessels of the heart (Fig. 283) come in contact so as to form 

 the letter V, the point of the V being toward the head of the embryo. 

 The arms continue fusing until a Y-shaped stem has been developed, 

 with the stem toward the head. 



Although the two tubes unite in the manner just mentigned, their 

 cavities remain distinct for a short time, the endothelial lining forming 

 two distinct cavities until a short time after the muscular walls have 

 fused. The muscular walls themselves are not complete on the dorsal 

 side for a short time, but as soon as the tubes have 

 thoroughly fused, the walls also complete their 

 function. 



It is the stem of the Y which forms the true 

 heart, the two arms being continuous with the large 

 vitelline veins which carry blood to the heart from 

 the vascular area. The caudal end of the heart is 

 then said to be venous, while the cephalic end is 

 known as the arterial end. 



At the thirty hour period the heart is a short 

 straight tube attached to the ventral wall of the 

 fore-gut or pharynx. The point where the vitelline 

 veins diverge is at the hindermost angle of the head- 

 fold. As the head-fold is pushed farther and farther 

 back, the straight portion of the Y is lengthened, 

 but as the tubular heart seems to grow more rap- 

 idly than does the place to which it is attached, it 

 is bent into a loop, with its convexity toward the 

 right side of the embryo. This looping is made pos- 

 sible by the fact that the heart has by this time lost 

 all connection with the wall of the fore-gut and re- 

 mains attached only at both ends. 



It is even before this period that the heart begins to beat, the pulsa- 

 tions beginning at the venous end and passing to the arterial end. In 

 fact, the beating began before any muscular differentiation could be ob- 

 served in the heart region. 



The cephalic end of the heart is known as the bulbus arteriosus. 

 The bulbus branches immediately into two narrow vessels, the aortic 

 arches, one passing upward on each side of the digestive tract to the 

 dorsal side of the embryo and then running tailward as the paired dorsal 

 aortae (Fig. 277). These vessels lie close to the notochord under the 

 somites, and extend as separate vessels almost to the tail, where a larger 

 branch than the vessel itself is given off from each. These two large 

 branches are the vitelline arteries, which carry the blood from the heart 

 back to the vascular area. 



au. 



Fig. 276. 



Anterior region of 

 day chick embryo, 

 optic vesicle ; crbl, cere- 

 bellum ; h, heart ; m.h., 

 vesicle of midbrain ; 

 med.obl., medulla oblon- 

 gata ; r.m., spinal chord ; 

 u.8., primitive segment ; 

 v.h'., primary vesicle of 

 the forebrain ; v.o.m., 

 omphalo-mesenteric vein ; 

 x, anterior wall of prim- 

 itive forebrain which 

 later expands into the 

 cerebrum. (After von 

 Mihalkovics.) 



