LEPIDOSIREN PARADOXA. 
97 
so also are the more prominent, more mesially situated 
trabeculae. Thus the interventricular septum is formed by 
the convergence of numerous muscular trabeculm, from the 
floor and sides of the ventricle, upon the auriculo-ventricular 
plug. With the further growth of the heart and the elon- 
gation and expansion of the ventricles in a caudal direction, 
the radiating mesh work of muscular bands becomes more 
complicated and denser (Text-fig. 18, F. S.), as well as more 
Text-fig. 17. 
Section through the heart at Stage 32. A. C. Auricular canal 
musculature. a. v. 1. Auriculo-ventricular ledge. av. r. 
Auriculo-ventricular ridge (plug) in auriculo-ventricular 
opening continuous dorsally with the pulmonary fold and 
ventrally with the ventricular musculature. L. A. Left 
auricle. P. /. Pulmonary fold. P. F. Opening of pulmo- 
nary vein. B. A. Right auricle. V. Ventricle. 
drawn out, until finally the septum acquires the solid char- 
acter of the adult condition. As this ventricular increase in 
size is a matter of peripheral expansion, the right and left 
ventricles are formed, not so much by the upgrowth of the 
septum as by the expansion backwards of the ventricles on 
either side of it. The somewhat unequal division of the 
ventricular cavity into a larger right and smaller left com- 
partment — due partly to the left-sided development of tho 
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