VALVES IN THE HEAKT OE THE CHICK. 
391 
tube, into the hinder or caudal end of which the omphalo-mesenteric veins open, while 
the branchial arteries emerge from the anterior or cephalic end. This tube soon in- 
creases in length and capacity, and becomes doubly bent upon itself, so that its general 
direction is at first from the entrance of the veins towards the head and ventral surface 
of the embryo, then bending backwards towards the caudal end, and from left to 
right, and then again bending towards the head and ending in the branchial arteries. 
As the contractile muscular tissue augments about the bends, the cephalic or left bend 
gradually becomes developed into a rudimentary auricle, the caudal or right bend, and 
the portion of tube between it and the cephalic bend, into the ventricle, while the con- 
siderable portion of the tube situated between the caudal bend and the branchial vessels 
remains free, or almost entirely so, from muscular tissue, and forms the bulbus aortse or 
truncus arteriosus (Plate XXXI. fig. 1). About the third day of incubation this large 
arterial trunk, into which the still single ventricle pours its blood, does not appear to be 
separated by any valvular apparatus whatever from the cavity of the ventricle. The 
vessel is twisted somewhat spirally after its exit from the ventricle, and the aperture of its 
transverse section is not everywhere circular, being oval or squarish at the bends. It is 
quite single and undivided throughout, and no ridges can be seen on its interior. It 
becomes gradually narrower as it approaches the point of division into the branchial 
arteries, in which it terminates abruptly. There is no valvular apparatus of any kind 
at the point of division into the branchial arteries, which are three on each side at this 
period. The first and second pair of branchial arteries have now disappeared ; the third, 
fourth, and fifth pair remain, and unite posteriorly to form the descending aorta. The 
deficiency of valves seems to be supplied by a considerable development of the elastic 
wall of the truncus arteriosus next the ventricle. This does not take place equally 
round the opening into the ventricle, but occurs principally on two opposite sides of the 
interior of the vessel, so that the circular aperture of communication between the ven- 
tricle and truncus arteriosus gradually becomes slit-shaped. 
The period at which the division of the truncus arteriosus into two vessels commences 
is, in the common fowl, about the 106th hour of incubation (four days and ten hours), or 
rather more than one-fifth (4f ) of the whole period of incubation, which is twenty-one 
days. There are three branchial arteries on each side, but they do not now come off 
abruptly from the end of the truncus arteriosus. Looked at sideways, the two anterior 
or upper pair of arches are seen to be separated from the posterior or fifth pair by a 
partition just extending a little way into the branchial end of the truncus arteriosus 
(Plate XXXI. fig. 2). This partition does not pass quite horizontally across the vessel, 
but is slightly inclined upwards from right to left, and extends a little further along the 
right side of the truncus arteriosus than it does along the left side ; it is in fact a pro- 
longation into the truncus arteriosus of the arterial wall between the fourth and fifth 
pair of vascular arches, and separates the rudimentary aorta from the rudimentary pul- 
monary artery, the little channel leading to the third and fourth pair being the first 
trace of a separate aorta, and that leading to the fifth pair the first trace of a separate 
