92 
JANE T. ROBERTSON. 
first, situated directly in front of the auriculo-ventricular 
opening-, and this relationship is permanent throughout 
development. 
Septa. — While these changes have been taking place in 
the various chambers, defining them externally and internally 
from one another, localised cellular proliferations occur that 
give rise to the various septa of the adult heart. The first 
of these structures to appear (Stage 27) — the auriculo-ventri- 
Text-fig. 14. 
Sagittal section through the heart at Stage 30, Auricle. 
A. C. Auricular canal, av.r. The auriculo-ventricular ridge. 
B. Bulbils cordis, with left ventral aorta above. }ili. Endo- 
deruial pharyngeal rudiment, 'pli. s. Pharyngeal sheath con- 
tinuous with av. r. S, V. Sinus veuosus. V. Yentricle. 
cular ridge — has already been partly described (p. 84) : 
it now remains to complete that description and to consider 
with it the development of the inter-auricular and inter- 
ventricular septa, as well as, to a certain extent, that of the 
pulmonary vein. The auriculo-ventricular ridge is intimately 
related to all of these important structures, forming, as it 
were, their common point of convergence. 
Pulmonary Fold. — The close relationship of the tAvo 
