832 



DEVELOPMENT. 



[CH. LVIII. 



op 



MB 



s.o 



a' 



649 ; this is viewed from below, so that the heart (h) receiving the 



two omphalo-mesenteric veins at its 

 hinder end is seen above the ali- 

 mentary canal, the yolk-sac having 

 been removed. 



The heart gives off anteriorly 

 the primitive aorta, which soon 

 divides into two ; these pass round 

 the blind anterior end of the ali- 

 mentary canal, and then pass back 

 along its dorsal aspect on each side 

 of the notochord, as seen in trans- 

 verse section (A, A) in fig. 648. 



The heart is at this time a 

 simple tube, but soon is divided 

 into a longitudinal series of cham- 

 bers which contract in the order 

 named from before backwards: (i) 

 the sinus venosus, where the veins 

 enter; (2) the auricle, which in 



pr 



Fig. 649. Embryo chick (36 hours), 

 viewed from beneath as a trans- 

 parent object (magnified). 



Fig. 650. Heart of the chick at the 45th, 65th, 

 and 85th hours of incubation, i, the venous 

 trunks ; 2, the auricle ; 3, the ventricle ; 

 4, the bulbus arteriosus. (Allen Thomson.) 



mammals fuses with the sinus to form a single chamber ; in 

 fishes and amphibians sinus and auricle are distinct ; then comes 

 (3) the ventricle, and (4) the commencement of the aorta, which 

 is called the aortic bulb. Later on the heart is twisted upon 

 itself in the way represented in fig. 650, so that the auricle 

 gets on the top of the ventricle, and the ventricle increases 

 in relative strength and size. 



Fig. 651 represents the primitive heart and vessels in outline. 



The omphalo-mesenteric veins (i, i) enter the auricle (2) ; then 

 come the ventricle (3) and aortic bulb (4) ; the two primitive 

 aortae arising from this pass forwards and then turn backwards 

 over the end of the fore-gut, and join together to form the dorsal 

 aorta (6) lower down ; the big branches (7, 7) which it gives off 

 are the two omphalo-mesenteric arteries to the yolk-sac. 



The smaller allantoic or umbilical arteries (8, 8) pass to the 



