42 Proceedings of the Royal Physical Society. 
puzzling to try to understand how the twisting of a tubular organ about its 
longitudinal axis has come about in evolution. One always feels inclined to ask 
the question whether the spiral twisting may not be a secondary result of 
some simpler process, such as mere uniform growth in length. An explanation 
of this kind seems to hold in the case of the apparent twisting of the intestine 
in the lower Fishes, which has arisen froma spiral coiling of the gut brought 
about by increase in length within a limited space. Such a question arises in 
the case of the spiral twisting of the conus and its derivatives in Amniota, 
and it is, I think, adequately met by assuming that the Lung-fishes retain an 
ancestral condition of the conus from which that of Amniotes (and Amphibians) 
has been derived. It is of interest to note that distinct vestiges of the double 
flexure of the conus appear to occur in Amniotes. Thus Greil has shown how 
in Lacerta the “bulbus” (i.e. conus) goes through a “ bayonet-shaped ” stage, 
and Hochstetter finds a still more strongly marked double flexure, like that 
of a Lung-fish, appearing as a transitory phenomenon in the heart of the 
crocodile.® 
What I suggest, therefore, is that in the common ancestral form from 
which Amphibians and Amniotes have been derived, there was present an 
elongated conus arteriosus folded in a double fold just like that which is still 
present in existing Lung-fishes, that during the evolution of the Tetrapods 
there has taken place a relative shortening of the conus (associated probably 
with the concentration of the valvular apparatus into a relatively short 
anteroposterior extent), and that the result of this shortening has been to 
straighten out the bends of the conus so as to convert it into a nearly 
straight tube, with a spiral twist about its long axis.* This has split up, of 
course, later in the higher Amniota into separate vessels twisted spirally 
round one another. 
I have taken it for granted that the anterior and posterior ends of the 
primitive cardiac tube are fixed in position, being firmly embedded in tissue, 
and this carries with it the necessary consequence, already perceived by 
Boas,* that any twist in the anterior portion of the tube must be neutralised 
by an equivalent twist in the contrary direction in the hinder part of the 
1 1903, Morph, Jahrb., Ba. xxxi. 
2 1906, Voeltzkow’s Reise in Ostafrika, Wiss. Ergebnisse, Bd. iv. 
’ It would appear from Boas’ papers that he looks upon the double flexure of the Lung- 
fish conus as having come about subsequent to the spiral twist, the already twisted conus 
” so as to produce the flexure (Morph, Jahrb., vi. 
pp. 326, 327, 332). Similarly the twisted conus of higher forms would be derived from a 
spirally twisted conus which had not developed the double flexure (Morph. Jahrb., vii. 
p. 511). 
' Morph, Jahrb.. vi. p. 332. 
having become “ zusammen-geschoben 
