304 



PROFESSOR J. COSSAR EWAHT. 



artery. These branches, together with many others from the sinus terminalis, form 

 an arterial network (fig. 34). From the network small veins proceed which unite 

 ultimately to form the large vitelline veins, by means of which the blood from the 

 yolk-sac is poured into the sinus venosus. The large left and small right vitelline 

 arteries and the large vitelline veins are seen in fig. 35. The general scheme of the 

 yolk-sac circulation is given in fig. 34, while fig. 22 and text-fig. 5 represent on a 

 larger scale the network formed by branches proceeding from the sinus terminalis. 



Text-fig. 5. — Portion of vitelline network near sinus terminalis. x 90. 



In the case of the rabbit and other mammals in which the lower polar region of 

 the blastocyst is never invaded by mesoderm, blood-vessels never occur beyond the 

 sinus terminalis ; when, however, as in the sheep, the mesoderm extends throughout 

 practically the whole of the lower portion of the blastocyst, no definite sinus termin- 

 alis appears. In the horse at the end of the third week, though a sinus has been 

 established, it does not, as in the rabbit, mark the limit of vascularisation, for, as fig. 22 

 and text-fig. 4 indicate, blood-vessels or blood-islands extend some distance beyond 

 the sinus. But by the end of the fourth week there are neither blood-vessels nor 

 blood-islands beyond the sinus terminalis ; hence the yolk-sac circulation in the horse 

 has even a different history from that of the pig, in which a temporary sinus is 

 followed by general vascularisation. 



