C>6 THE SECOND DAY. [CHAP. 



ence however is in the course of events carried farther hack, though the 

 lower diverging folds (the lower limbs of the X) disappear, the thickening at 

 the point remains and increases. In a short time, consequently, we do find 

 a thickening of the mesoblast in the under-wall of the foregut just in front 

 of the point of divergence, which thickening is continuous like an inverted j^, 

 with two thickenings reaching down the diverging folds behind the point 

 of divergence. 



This \-shaped thickening becomes hollow by a transformation of its 

 central cells ; the single cavity in front is the cavity of the heart, and the two 

 diverging cavities behind, with which it is continuous, are the canals of the 

 omphalo-mesaraic veins. 



As development proceeds, and the point of divergence is carried still farther 

 and fartlier back, the heart increases in length step by step at the expense 

 of the continually coalescing omphalo-mesaraic veins. 



The coalescence of the mesoblastic thickening which forms the walls of the 

 veins precedes that of their canals, consequently in sections taken at parti- 

 cular points we meet with two cavities invested by one wall. This is probably 

 what was seen by the observers who have described the heart as being formed 

 as a double tube which afterwards became single. 



The front end of the cavity of the heart is continuous with canals similarly 

 formed in the mesoblast of the foregut by the solution of certain ceils. These 

 are the canals of the aorta?. 



At first the substance of the heart is along its whole length adherent to and 

 indeed a part of the underwall of the foregut. Sub-equently it becomes free in 

 its middle poition, the arterial and venous ends alone remaining attached. 



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 elements. 



6. To provide channels for the fluid thus pressed by the 

 contractions of the heart, a system of tubes has made its 

 appearance in the mesoblast both of the embryo itself and of 

 the vascular and pellucid areas. In front the single tube of 

 the heart 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 protovertebrse (Figs. 18, Ao, 20, Ao). 

 About half way to the hinder extremity each gives off at right 

 angles to the axis of the embryo a large branch, the omphalo- 

 mesaraic artery (Fig. 23, Of A.), which, passing outwards, is 



