LEPIDOSIHEN PARADOXA. 
81 
but tliis condition is transitory, though the flattened oval 
shape ot* the lumen of the heart-tube — attributable probably 
to the spherical surface of the yolk and the close approxima- 
tion of the embryo against it — persists for a considerable time. 
This characteristic arrangement of the rudiments of the 
vitelline veins, heart and ventral aortm is probably due to the 
presence of the yolk. Instead of being extended in the long 
axis of the embryo with the rudiments of the great vessels 
appearing far apart at either end of it, the heart-rudiment 
has suffered an approximation of its cranial and caudal 
extremities, while the intervening part has become folded on 
itself and projects ventrally over the yolk. This arrangement, 
therefore, is merely an expression of the relations of embryo 
and yolk peculiar to Lepidosiren. 
Shape and Attachments of the Heart. — From the 
first the heart-rudiment is much flattened between the head of 
the embryo and the yolk, and the comparative approximation 
of its two ends is maintained throughout development. This, as 
alread}^ suggested, is determined by the presence of the yolk 
and by the flattened, laterally expanded shape of the pericar- 
diac space, both of which conditions, of course, are inter- 
dependent and persist for a comparatively long time. These 
factors also account for the vertical position of the heart, 
relative to the long axis of the embryo, during its earlier 
stages of development. 
When the mesoderm plates meet ventrally in the cardiac 
region they fuse rapidly, and the extremely short ventral meso- 
cardium disappears almost as soon as formed (Stage 24 +, 
Text-fig. 5b, V. mes.). The heart now grows rapidly in length, 
and as its anterior and posterior ends remain relatively fixed 
it projects ventrally more and more into the narrow pericar- 
diac space and forms a loop, flattened antero-posteriorly 
(Text-fig. 8). This rapid growth of the heart-tube, combined 
with the peculiar shape of the cavity in which it is placed, 
causes a degree of twisting, the immediate effect of which is 
to free the loop from the dorsal mesocardium, so that now 
the heart has a complete myocardiac covering, only the dorsal 
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