86 
JANE J. EOBERTSON. 
axis of the embryo, till, from being practically at right angles 
to, the heart loop is once more parallel with, the length of 
the embryo (PI. 5, figs. 5, 6 and 7). Throughout it must be 
remembered that the anterior and posterior ends of the heart, 
are fixed, and therefore whatever rotation occurs affects only 
the loop of the tube, and further, that the second rotation 
is merely a recovering of the original position of the primitive 
endothelial rudiment before its peculiar environmental rela- 
tions compelled it to adopt a transverse position. Ultimately, 
therefore, the amount of twisting displayed by the heart as a 
whole is negligible. Also as the yolk disappears, the heart, 
from being vertical (Text-figs. 9 and 12), becomes more and 
more horizontal in position, the ventricular axis coming to 
form a comparatively acute angle with the common axis of 
sinus and auricle (PI. 5, fig. 1). Thus in Lepidosiren, 
owing to the exigencies of the yolk, the definitive position of 
the heart is not finally assumed till a fairly late stage in 
development is reached, which renders a description of its 
changing anatomical relations peculiarily difficult. In the 
meantime, however, tor the sake of simplicity, the heart will 
be considered as vertical to the long axis of the embryo, the 
position which is maintained more or less till development is 
practically complete, and with its loop rotated into the defi- 
nitive antero-posterior position. The terms anterior^’ and 
posterior thus express in the earlier stages relationships 
that in the adult are ventral and dorsal respectively. 
The auricular part of the heart now expands dorsally and 
laterally but more especially laterally, and comes to bulge on 
either side i*ound the conus (compare Text-fig. 9 A, b and c, A) ; 
it also gradually expands on either side of the sinus venosus 
ventral to the ducts of Cuvier, and overlaps the unexpanding 
auricular canal anteriorly and laterally. 
The posterior auricular wall, however, on which the auriculo- 
ventricular ridge has appeared, takes little part in this general 
enlargement, with the result that it remains comparatively 
short, and the sinu-auricular and auriculo-ventricular open- 
ings, which are situated at either end of it, remain com- 
