IV.] 



THE HEART. 



87 



unpaired portion in front and two diverging limbs be- 

 hind. The unpaired portion is the true heart, while the 

 diverging limbs are the vitelline veins already spoken 

 of (Fig. 28, sp). While the changes just spoken of 

 have been taking place in the external form of the 

 heart, its internal parts have also become differentiated. 

 A cavity is formed in each of the halves of the 

 heart before even they have coalesced. Each of these 

 cavities has at first the form of an irregular space 



FIG. 30. 



TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE 

 HEAD OF AN EMBRYO CHICK OF THIRTY HOURS. 



hb. hind-brain ; vg. vagus nerve ; ep. epiblast ; ch. notochord ; 

 x. thickening of hypoblast (possibly a rudiment of the sub- 

 notochordal rod) ; al. throat ; lit. heart ; pp. body cavity ; 

 so. somatic mesoblast ; sf. splanchnic mesoblast ; hy. hypo- 

 blast. 



