LEPIDOSl HKN PA R ADOXA. 
95 
wards across the dorsal surface of the auriculo-ventricular 
plug into the auricular cavity. By the fusion of its anterior 
margin with the developing muscular trabecuhn of the intei-- 
auricular septum (Text-fig. 15 c aud d, A. S., p. v.) this fold 
comes to form part of that structure. With the development 
of the pulmonary vein the left auricle and ventricle expand 
rapidly, approximating more to the dimensions of the com- 
partments on the right side of the heart. 
In teranricular Septum. — Simultaneously with the 
appearance of the pulmonary fold (Stage 31), a little growth 
of the endothelium of the auricular roof takes place across 
that part of it that may be said to be pinched in the narrow 
space between the bulbus cordis and the sinus venosus. The 
little endothelial fold (Text-fig. 15b, A. S.), the rudiment of 
the interauricular septum, arches across from the posterior 
wall of the bulbus to the anterior termination of the rudimen- 
tary pulmonary fold and the auriculo-ventricular ridge, and 
there fuses with them (Text-fig. 15a, b, c and d, A.S.), 
thus in its turn also coming into relation with the auriculo- 
ventricular ridge. Later, fine muscular strands develop in 
and about this little septum, forming an irregular meshwork 
of fibres placed more or less parallel to the long axis of the 
heart, but it does not appear to attain to any marked degree 
of development till after the adult stages are reached. 
The situation and development of the interauricular 
septum as well as its relations to the auriculo-ventricular ridge 
(plug) are identical with those of the similar septum in Urodeles 
sinn-anricular junction. The inter-auricular sejDtum {A. S.), 
the wall of the pulmonary vein (p. v.) and the auriculo-ventri- 
cular ridge {av. r.) are all three continuous in this section. 
E. Posterior to the sinu-auricular junction, f. Through the 
sinus venosus posterior to fig. E. G. Through the sinus veno- 
sus posterior to fig. f, Note continuity at this point of the 
column of cells {p. v.) on the dorsal wall of the sinus in which 
the pulmonary vein will develop, with the tissue of the iDharyn- 
geal and lung sheath (p/t. s.). d. Inter-auricular septum. 
av. r. Auriculo-ventricular ridge. B. Bulbus. L. A. Left 
auricle, p. v. Column of cells situated anteriorly on the left 
and posteriorly on the dorsal wall of the sinus venosus, in which 
the pulmonary vein will develop. pJi. s. Pharyngeal sheath. 
R. A. Bight auricle. S. V. Sinus venosus. V. Yentricle. 
