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C. V. MORRILL 



a solid quadrilateral mass. A cap of developing muscle covers 

 its posterior border (upper in the figure) and extends anteriorly 

 for some distance along its lateral surfaces. This muscular 

 mass is continuous with the musculature of septum I and, at a 

 lower level (not shown in the figure), with that of the two sinus 

 valves which, as mentioned, blend with the root of the septum. 

 A careful study of serial transverse sections shows that the 

 musculature of this part of septum II is derived from that of 





Fig. 4 Transverse section of the heart region of a l.l.L' mm. embryo at the 

 level of the root of septum II (histological structure semi-diagrammatic). A., 

 dorsal aorta; A.d., right atrium; A.s., left atrium; En.c, fused endocardial cush- 

 ions; S.in., interventricular septum; Sp.in., spatium intersepto-valvulare; S.I., 

 septum I; S.IT, septum II; V.car.c.d.. and V.car.c.s. right and left common 

 cardinal veins. Magnified 25 diameters. 



