506 
J. GRAHAM KERR. 
There is first what may be called the primary torsion of 
the lung, which is illustrated by the camera drawing in text- 
figs. 9 and 10, A — D. During the early stages of its develop- 
ment the lung rudiment, while increasing in length, describes 
a spiral curve ^ round the oesophagus. Starting from the 
mid-ventral glottis it grows first ventralwards, tailwards, 
and towards the right side; then dorsalwards and tailwards; 
and, finally, tailwards and towards the mesial plane, until it 
attains its mid-dorsal position over the gut, after which it 
grows directly tailwards. Now, during the spiral part of this 
course, in addition to changing its position relative to the gut 
(being first ventral, then on the right, and finally dorsal), 
the lung undergoes what I have called the primary torsion — 
torsion about its own long axis in a counter-clockwise 
direction, as seen from the tailward direction. This torsion 
takes place through 180°, so as to cause a complete reversal 
in position of the hinder part of the lung rudiment at this 
stage, its morphologically ventral aspect becoming dorsal, its 
originally right side becoming left. The process of torsion 
is continued still further, however, in the majority of speci- 
mens, for it may be as much as 30° or 40°. The result is that 
in such specimens, when the tip of the lung begins to 
bifurcate, the (actual) left lung is seen to be considerably 
displaced towards the ventral side, as compared with the 
right (see text-figs. 8, B and c). This difference in level soon 
becomes corrected by processes of differential growth, in 
which a secondary torsion takes place, so that the two lungs 
are brought to the same level. It will be seen that this 
secondary torsion is in a direction the reverse of the primary 
torsion (i. e. it is clockwise, as seen from the tail end).^ The 
' Only visible in occasional individuals. More usually the oesophagus 
is sufficiently out of the way to the left side to render the cuiwing no 
longer necessary. 
" The primary torsion of the oesophagus is clearly of the type which 
would naturally be associated with a spiral coiling of a dextral type 
like that of the inid-giit, while the secondary torsion may express a 
tendency to return to the original condition. The possibility is 
