DO 
JANE I. 110 EE RTS ON. 
tlie proximal part of the bulbus round to the left (PL 6 , 
tigs. 5^ 6 and 7). This, combined with the presence of the 
expanding auricles, probably assists in determining the 
bulging of the middle pai*t mainly in an anterior direction 
and to the left. The bnlbns has now attained its adult form, 
and lies cushioned against the anidcles, appearing to indent 
them, with its posterior (dorsal) wall very intimately related 
to their anterior (ventral) surfaces (PI. 5, fig. 7). 
Text-fig. 13. 
Horizontal section tbrongh the heart at Stage 32. S. V. Sinus 
venosus. r. S. A. Right sinu-auricular fold. R. A. Right 
auricle. B. C. Bulluis cordis. A. S. Auricular septum. L. A. 
Left auricle. Pf. Incipient pulmonary fold. P. V. Pul- 
monary vein. 
By Stage 32 the demarcation of the various chambers 
from one another is achieved, and their various orifices more 
or less clearly defined. 
The sinn-anricular opening remains, as before, a circular 
aperture opening into the auricular chamber on the right 
and having the auriculo-ventricular ridge with the develop- 
ing pulmonary fold (Text-fig. 13, P. /.) on the left. The 
bulging of the right auricle along the right side of the sinus 
venosus, as already mentioned, gives rise to a constriction 
externally between the two chambers, that is represented in 
the interior of the heart by a fold guarding the right side of 
