214 H. D. Senior. 



and the roof by the ventral surface of the splanchnic mesoderm. 

 The roof is entirely lined by vascular endothelium. The floor, 

 having no endothelial lining, presents the naked periblast of the 

 yolk to the blood-stream passing over it. Although the mid-sagittal 

 plane on the embryo would fall (except near the anterior pole of 

 the yolk, see Figs. 21 and 22) entirely within the tunnel, it would 

 separate the latter into two unequal parts, of which the left would 

 be greater than the right. 



The supravitellins sinus, consisting of the tunnel described above, 

 receives blood from the short vena rcveliens of the liver and dis- 

 charges it into the, somewhat roomy, chamber embracing the anterior 

 pole of the yolk. The liver, at this time, is situated dorsal to the 

 posterior pole of the yolk (see Fig. 2). The chamber at the anterior 



Fig. 2.— Diagram indicating tlie arrangement of the principal blood-channels 

 of tbe recently hatched shad of which the heart is shown in Fig. 1. X 10 

 diams. 



Arteries black ; veins, and supravitelline sinus, stippled. 



pole of the yollv, which corresponds in position with the future sinus 

 venosus, is bounded as follows: Posteriorly by the naked anterior 

 pole of the yolk; elsewhere by the portion of the heart-wall im- 

 mediately succeeding the atrium, which is later to form the anterior 

 wall of the sinus venosus. Venous blood enters the chamber from 

 the jugular and cardinal veins and from the supravitelline sinus, and 

 leaves it by passing through the orifice leading into the atrium. 



The relation of the yolk to the vascular system is somewhat as 

 follows: While it converts the partially formed sinus venosus and 

 the (subintestinal) supravitelline sinus into a closed passage, capable 

 of retaining and transmitting blood, it, at the same time, delays 

 the formation of the posterior wall of the sinus venosus and of the 

 vessel later to be formed by the vascular endothelium lining the 

 supravitelline sinus. 



