THE CIRCULATORY SYSTEM 
In no other system of organs does the fundamental law of 
biogenesis find such wide application as in the circulatory, and 
to go into details concerning it would be merely to repeat 
what has been often said before. Attention may therefore be 
confined to the following facts. 
THE HEART 
The heart arises (ed., Fig. 31, A), at an early embryonic 
stage, far forwards in the cervical and indeed in the cephalic 
region. This recalls its position in adult Fishes and Amphibians. 
The comparison with these animals is the more fully justified, 
in that the heart of the early human embryo, like that of the 
lowest Anamnia, has throughout a single lumen, and its further 
differentiation is gradually undergone in correspondence with the 
phylogenetic development of the organ. 
The structure of the heart, originally very simple, soon 
becomes complicated, but even then certain peculiarities of the 
right auricle point back to the condition found in the Amphibia. 
These are, for example, the inconstant vestiges of valves at the 
opening of the left vena cava superior (Thebesian valve), and 
the almost constant remains of the valves of the sinus at that of 
the vena cava inferior (Eustachian valve). The same applies to 
the traces of the incorporation of the sinus venosus and of the 
pulmonary veins into the opposite divisions of the atrium 
(auricles). In short, Comparative Anatomy furnishes not only 
interesting parallels with, but an explanation of the various stages 
in the Ontogeny of the heart of the higher Vertebrata. There 
are, however, some conditions which occur in the Mammalian 
heart, especially during the early periods of its development, 
which cannot be explained by inheritance, but which have arisen 
secondarily through adaptation; among the chief of these are the 
