]02 THE SECOND DAY. [OUAl'. 



opening one on each side of the hind-brain (Fig. 27, 

 au. p). 



Heart. We left the heart as a fusiform body 

 sUghtly bent to the right, attached to the under wall 

 of the foregut by the mesocardium. The curvature 

 now increases so much that the heart becomes almost 

 CO -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 say that the free intermediate portion is by 

 its own growth bent downwards, backwards, and some- 

 what 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 

 splanchnopleure folds which marks the union of the 

 vitelline 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 co ; the other, the 

 arterial bend, the left-hand curve of the W. 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 here- 

 after grow into the ventricles. 



Vascular system. The blood-vessels, whose origin 

 during the first half of this day 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 



