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PROFESSOR J. COSSAR EWART. 



heart; (3) the elongation and folding of this heart so that it assumed the form of 

 the letter "S"; and (4) its constriction into four more or less distinct chambers. 



laa. 



pcv 



Text-fig. 7. — Diagram of heart and blood-vessels. v.v., right vitelline vein; s.v., sinus venosus ; 

 at, atrium; v., ventricle; b., bulbus arteriosus; l.a.a., first aortic arch; ll.a.a., second 

 aortic arch ; l.a., left aorta ; r.a., right aorta ; a.o., dorsal aorta ; l.v., left vitelline artery ; 

 r.v., right vitelline artery; pi. , plexus in allantoic mesoderm; a.c, anterior cardinal vein; 

 p.c.v., posterior cardinal vein ; c.d., duct of Cuvier ; u.v., umbilical vein ; l.b.a. to IV.b.a., four 

 branchial arches ; l.b.p. to IV. b.p., four branchial pouches. 



The vitelline veins, which play so important a part during the earlier weeks, are seen 

 in figs. 34, 35, and 43 approaching the embryo. Fig. 43 also shows the lateral 

 portions of the sinus venosus in the pleuro-peritoneal canals. The intimate relation 

 of the sinus to the fore-gut is made evident by fig. 42. The sinus venosus, though 



