IV.] THE CIRCULATION. 79 



aorta and by its venous end, but with its intermediate portion 

 quite free. The curvature now increases so much that the 

 heart becomes almost m -shaped, the venous portion being 

 drawn up towards the head so as to lie somewhat above (dorsal 

 to) and behind the arterial portion. (It would perhaps be more 

 correct to pay that the free intermediate portion is by its 

 own growth* bent downwards, backwards, and somewhat to 

 the right, while the venous root of the heart is at the same 

 time continually being lengthened by the carrying back of 

 that "point of divergence " of the splanclmopleure folds which 

 marks the union of the omphalo-mesaraic veins into a single 

 venous trunk). The heart then has at this time two bends, 

 the one, the venous bend, the right-hand curve of the Ul ; 

 the other, the arterial bend, the left-hand curve of the ^ • 

 The venous bend which, as we have said, is placed above 

 and somewhat behind the arterial bend, becomes marked by 

 two bulgings, one on either side. These are the rudiments 

 of the auricles, or rather of the auricular appendages. The 

 ascending limb of the arterial bend soon becomes conspicuous 

 as the bulbus arteriosus, while the rounded point of the 

 bend itself will hereafter grow into the ventricles. 



19. The blood-vessels, whose origin during the first half 

 of this clay has been already described, become during the 

 latter part of the day so connected as to form a complete 

 system, through which a definite circulation of the blood is now 

 for the first time (consequently some little while after the 

 commencement of the heart's pulsation) carried on. 



The two primitive aortas have already been described as 

 encircling the foregut, and then passing along the body of 

 the embryo immediately beneath the protovertebra? on either 

 side of the notochord. They are shewn in Fig. 20 a.o in section 

 as two large rounded spaces lined with spindle-shaped cells. 

 At first they run as two distinct canals along the whole 

 length of the embryo ; but, after a short time, unite at some 

 little distance behind the head into a single trunk, which 

 lies in the middle line of the body immediately below the 

 notochord (Fig. 39). Lower down, nearer the tail, this 

 single primitive trunk again divides into tw T o aorta?, which, 

 getting smaller and smaller, are finally lost in the small 

 blood-vessels of the tail. At this epoch, therefore, there are 

 two aortic arches springing from the bulbus arteiiosus, and 



