IV.] THE VASCULAR SYSTEM. 89 



some time separate, and cause the cavity of the heart to 

 be divided into two tubes even after its two halves have 

 to all appearance completely coalesced 1 . 



Soon after its formation the heart begins to beat ; 

 its at first slow and rare pulsations beginning at the 

 venous and passing on to the arterial end. It is of some 

 interest to note that its functional activity commences 

 long before the cells of which it is composed shew any 

 distinct differentiation into muscular or nervous ele- 

 ments. 



Vascular system. To provide channels for the 

 fluid thus pressed by the contractions of the heart, a 

 system of tubes has made its appearance in the meso- 

 blast both of the embryo itself and of the vascular and 

 pellucid areas. In front the single tube of the bulbus 

 arteriosus bifurcates into two primitive aortce, each 

 of which bending round the front end of the foregut, 

 passes from its under to its upper side, the two forming 

 together a sort of incomplete arterial collar imbedded 

 in the mesoblast of the gut. Arrived at the upper side 

 of the gut, they turn sharply round, and run separate 

 but parallel to each other backwards towards the tail, in 

 the mesoblast on each side of the notochord immediately 

 under the mesoblastic somites (Figs. 32, Ao, 34, ao). 

 About half way to the hinder extremity each gives off 

 at right angles to the axis of the embryo a large branch, 

 the vitelline artery (Fig. 36, Of, A.}, which, passing 

 outwards, is distributed over the pellucid and vascular 

 areas, the main trunk of each aorta passing on with 

 greatly diminished calibre towards the tail, in which it 

 becomes lost. 



1 This is not shewn in the diagram, Fig. 31, A. 



