354 



C. V. MORRILL 



>r\ ^ ,/V cares \ 



''^^S?L»V'*C> 



W "^i---^ 



Figs. 1 to 3 Transverse sections of the h^art region of a 7.9 mm. embryo 

 (histological structure semi-diagrammatic). Figures 1 and 3, above and below 

 the level of ostium I respectively; figure 2, through ostium I. A., dorsal aorta; 

 A.d., right atrium; ^.s., left atrium; En.s., upper endocardial cushion; En.i., 

 lower endocardial cushion; O.I, ostium I; S.in., interventricular septum; S.I., 

 septum I; S.II, septum II; Sn.d., right sinus horn; Sp.in., spatium intersepto- 

 valvulare; V.car.c.cL, and V.car.c.s., right and left common cardinal veins; 

 V.r.d., right sinus valve; Vv.v., sinus valves. Magnified about 2.5 diameters. 



