394 
DE. M. TONG-E ON THE DEVELOPMENT OF THE SEMILTJNAK 
the vessels are quite distended. This is shown in Plate XXXI. figs. 14 & 15 ; the outer 
valve-rudiments are there just beginning to appear. 
At the 130th hour of incubation (five days ten hours) we find that the pyramidal 
ridge on the posterior surface of the truncus arteriosus has become more developed at 
every part of it, being very strongly marked near the valves, only slightly so in the 
middle portion of the vessel, but again becoming prominent below, where it terminates 
by suddenly narrowing, and being slightly twisted on itself slants into the edge of the 
left lip of the opening into the ventricle, passing downwards and forwards. The channel 
for the pulmonary blood is represented by the deep groove that passes down the left 
side of the ridge, and terminates in the wide and sloping surface of the left lip of the 
slit. The groove on the right side of the ridge forms the channel for the aortic blood, 
and passes almost directly downwards to the posterior end of the slit. Thus the aortic 
groove passing directly downwards, the pulmonary groove, which is at first situated to 
its left, gradually twists round the lower end of the ridge separating the two, and ex- 
panding out below opens obliquely into it. The figures 16, 17 & 18 are drawings 
of a section of the truncus arteriosus at this period, which includes the valves and the 
descending septum, and shows pretty clearly the mode in which the vessel is dividing 
and the stage of development attained by the valves. In Plate XXXI. figs. 17 & 18, 
the shape of the rounded pyramidal ridge is well seen. A prominent convex ridge 
separates the two free convex and sloping sides of the pyramid. Below it is almost 
lost on the wall of the vessel, whose canal is almost circular a short distance below the 
valves. Above it projects forwards into the vessel for about two-thirds of the diameter 
of the vessel at the apex of the fork of the descending septum, and it represents what 
was originally the right leg of the fork. The other leg of the forked septum is repre- 
sented by a slighter ridge, which bounds the rudiment of the anterior aortic valve on 
the left, and is very soon lost on the wall of the vessel. The fork of the septum grows 
down the vessel along the course of the prominent posterior ridge, and on the anterior 
aspect of the vessel along the slighter ridge to the left of the anterior aortic valve and 
the adjacent part of the groove separating the anterior aortic from the anterior pulmo- 
nary valve. This anterior portion of the fork is, as we have seen, very slightly marked, 
and is very soon lost on the wall of the vessel. The posterior ridge is, on the contrary, 
already distinct along the whole of the truncus arteriosus, though most so above and 
below. At no time during the division of the vessel does the anterior portion of the 
fork of the septum extend for more than a very short distance along the anterior surface 
of the artery ; so that the division cannot be correctly described as occurring by the 
formation of two longitudinal ridges situated opposite to one another, and their subse- 
quent growth together throughout their whole length, as Bathke states to be the case, 
but must be described as taking place by the growth down the vessel of a forked septum 
which extends into the vessel nearly horizontally from between the fourth and fifth pair 
of branchial arteries, and which has already separated the rudimentary aorta and pulmo- 
nary artery before any ridges , properly so called, have appeared in the vessel, and which 
