380 BRADLEY M. PATTEN 
of blood is not established until the 16-somite stage. The fact 
that circulation does not begin until after the bending of the 
heart is well advanced excludes inequality of blood-flow from 
consideration as causative factor ontogenetically. There is 
still the possibility that the bending of the heart to the right is a 
recapitulation of development in some ancestral form where 
lateral inequality of the circulation had become established 
prior to the bending of the heart, but it would be equally plausible 
to urge that the bending of the heart to the right was primary, 
and that the left omphalomesenteric vein became secondarily 
enlarged owing to the opposition of less resistance to the dis- 
charge of its blood. 
It has also been suggested that, owing to the direction of the 
torsion of the embryo’s body, the heart tube is free to expand to 
the right, while it would be obstructed on the left by the swing- 
ing of the left side of the body wall toward the yolk. Again we 
encounter a disregarded time factor. The heart bending is ini- 
tiated before there is any indication of torsion in the embryo. 
There is undoubtedly correlation between the two processes in 
the sense that the development of the heart would be mechani- 
cally impeded, if not stopped, by torsion of the embryo in the 
opposite direction. Here also it might be maintained that the 
heart bend itself is the primary factor and that the direction of 
embryonic torsion follows it as a necessary consequence. Cer- 
tain it is that the bending of the embryonic heart to the right is 
not peculiar to forms in which torsion is conspicuous. The heart 
bend is the more deep seated phylogenetically. It occurs in 
the vertebrate stock as far back as the elasmobranchs (Hoch- 
stetter, 06) and Dipnoi (Robertson, 713), and is a characteris- 
tic feature of heart development in Amphibia (Rabl, ’87). One 
would scarcely expect to work out the primary causative factors 
of such a long-established process entirely from the ontogeny of 
forms as far up the scale as birds. 
As has already been stated, the ventral mesocardium has 
disappeared by the time the bendicg of the heart becomes ap- 
parent. ‘The dorsal mesocardium, which is complete when the 
bending process begins, soon ruptures in the midheart region 
aes 
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