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 ventricle 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 aortæ (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. Kölliker 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 development 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 development 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.