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X. Observations on the Development of the Semilunar Valves of the Aorta and Pulmo- 
nary Artery of the Heart of the Chicle. By Morris Tonge, M.A., M.D. Commu- 
nicated by Dr. Beale, F.JR.S. 
Received March 24, — Read April 30, 1868. 
In the accounts of the development of the heart of vertebrate animals given by various 
embryological writers, we find an apparently clear description of the mode in which the 
permanent aorta and pulmonary artery are formed by the longitudinal division of a 
single large vessel, the truncus arteriosus, into two vessels. The truncus arteriosus is, 
as is well known, the large arterial trunk that commences in the originally single ven- 
tricle of the heart, and terminates by splitting up into the branchial arteries. It conveys 
the whole of the blood from the ventricle to the system of the embryo, and is also known 
by the name of bulbus aortas (see Plate XXXI. fig. 1). In studying the descriptions given 
by different authors of its division into two vessels, it appeared to me very strange that 
with such a clear description of the mode of division nothing at all, or only very little, 
should be said about the mode of development of the semilunar valves attached to the 
commencement of these vessels. Kolliker is the only author I have been able to find 
who makes any mention of their mode of development, and his account, which I shall 
presently quote, is very brief and unsatisfactory. Nowhere have I found any drawings 
of these parts in their rudimentary state. I was therefore obliged to conclude that very 
little was known about this point, probably in consequence of the difficulty of accurately 
examining such minute parts at an early period of development, and I was hence led to 
attempt the observations recorded in the present paper. They were made during the 
years 1865, 1866, and 1867, on the embryo of the common domestic fowl, artificially 
incubated. Though not nearly as complete as I could have wished them to be, they 
nevertheless demonstrate certain new and interesting facts connected with the develop- 
ment of the semilunar valves, and the formation of the aorta and pulmonary artery in 
the bird’s heart. These appear to me to be valuable, as possibly throwing light on some 
of the congenital malformations of this part of the heart. In working at the develop- 
ment of the semilunar valves, I was also obliged to examine very closely into the mode 
of division of the truncus arteriosus into two vessels, and found that the manner in which 
it becomes divided differs from that usually described to occur in some very important 
particulars. The development of the semilunar valves is so closely connected with the 
process of division of the truncus arteriosus that I have found it best to unite the 
description of each stage of the one with that of the corresponding stage of the other. 
As the satisfactory examination of these minute parts is difficult, and requires a pecu~ 
3 F 2 
