Note on the Morphology of the Heart of Vertebrates. 43 
tube. Unfortunately, in attempting to examine into this part of the problem, 
we are handicapped by the fact that the endocardiac swellings of the atrio- 
ventricular part of the tube are in the form rather of isolated cushions than 
of longitudinal ridges; but we are probably fairly safe in assuming that the 
left-handed twist, which must necessarily be present in this part of the heart, 
even though not visible, came about in evolution by a similar process of 
folding to that by which we have explained the twisting of the conus region. 
To what extent this primary flexure of the atrioventricular portion of the 
heart is represented by the flexure which occurs during early stages of 
ontogeny is a question of much interest, but one which is not easy to answer 
definitely on account of the difficulty already alluded to. 
(Issued separately, 21st May 19138.) 
