Note on the Morphology of the Heart of Vertebrates. 39 
conus which has just been defined is clearly an apparatus for the prevention 
of regurgitation of blood into the ventricle during the diastole of the latter. 
The systole of the ventricle is followed by that of the conus, the effect being 
to jain together the endocardiac ridges so as to obliterate the lumen, and in 
this way effectively to prevent any sucking back of the blood when the 
ventricle again dilates. This primitive condition of the conus persists 
among the Gnathostomata at the present day only in the embryos of various 
Vertebrates. 
In tracing out the further evolution of the primitive conus we may 
distinguish two main processes at work. Firstly, we see a continuation of that 
process which, in the first instance, led to the marking off of the primitive 
heart from the rest of the ventral vessel—the greater localisation of the 
power of rhythmic contraction. The primitive conus, the front part of the 
primitive cardiac tube, loses its rhythmic contractility from before backwards, 
its striated muscle becoming replaced by smooth muscle fibres, and, in fact, 
its general histological structure becoming like that of the ventral aorta.’ 
Secondly, we see that the physiologically elaborate neuromuscular 
apparatus for preventing regurgitation from ventral aorta into ventricle 
becomes replaced by a simple, purely mechanical and automatic, and therefore 
more reliable, arrangement of pocket- (semilunar) valves. In Elasmobranchs 
it can still be seen in ontogeny how each longitudinal ridge becomes 
segmented up to form a row of pocket-valves. In Crossopterygians 
(Polypterus—and no doubt in Actinopterygian Ganoids also, although in them 
the ontogenetic development has not yet been worked out) a similar process 
takes place, each longitudinal ridge being represented by a longitudinal row 
of pocket-valves in the adult. 
One of the points of this note is to accentuate the morphological import- 
ance of these ridges or rows of valves, as marking morphologically longitudinal 
lines on the conus wall and thus affording invaluable landmarks in studying 
the processes of folding and twisting which this part of the heart has under- 
gone during the evolution of the higher groups of Vertebrates. Individual 
ridges may, in particular animals, be discontinuous. In the Amniota, for 
example, each ridge is represented either temporarily or during the whole of 
its existence by two distinct rudiments, one anterior and one _ posterior. 
Whether or not this discontinuity is, as I am inclined to believe, secondary, 
1 The observed facts are susceptible of being interpreted, as they are by Greil (1903, 
Morphol. Jahrb., xxxi., etc.), as being due to an actual backerowth of the ventral aorta or 
truncus at the expense of the conus. Looking to the fact, however, that the cardiac tube is 
firmly fixed in position at its point of exit from the pericardiac cavity, it seems to me 
reasonable to regard the structures lying between this and the ventricle as homologous 
in the various groups whatever their histological condition in the fully developed state, 
