330 Dr. Tonge on the Development of the Semilunar [Apr. 30 



pulmonary artery, and I havQ not been able to discover any observations 

 later than his. After speaking of the formation of the aorta and pul- 

 monary artery by the division of the truncus arteriosus into two vessels, 

 this being, as is well known, the large single arterial trunk conveying the 

 blood from the rudimentary ventricle into the branchial arteries, he 

 says*, "Simultaneously with the division the semilunar valves also be- 

 come developed, and I saw them already present in both arteries in an em- 

 bryo of the seventh week. They are, however, at first nothing but hori- 

 zontally projecting crescentic growths of the middle and of the epithelial 

 coats by which the lumen '\- at this spot receives the form of a three-rayed 

 star. At what time they first become visible as distinct pockets I have not 

 yet investigated." 



The division of the truncus arteriosus is described by Rathke as occur- 

 ring in birds and mammalia by the formation on its interior of two oppo- 

 sitely situated longitudinal ridges, which then grow together throughout 

 its whole extent and completely divide the vessel into two lateral halves, 

 one representing the commencement of the aorta, and the other that of the 

 pulmonary artery. Though the semilunar valves are said by Kolliker, and 

 quite correctly, to develope simultaneously with the division, he gives no 

 information about the manner in which they are connected with it, or the 

 part of the vessel in which they originate, and nowhere are any drawings 

 given of them in their rudimentary state. I was hence led to conclude 

 that very little was known about this point, and to make the observations 

 the results of which are here recorded. They seem to me valuable, as 

 throwing light on some of the congenital malformations of this part of the 

 heart. They were made during 1865, 1866, and 1867, on the embryos of 

 the common fowl, and I have had no opportunity of investigating human 

 or other mammalian embryos with reference to this point. But from the 

 great likeness between the hearts of birds, mammalia, and man at different 

 periods of their development, it seems pretty certain that the arterial 

 semilunar valves in man and mammalia generally must pass through the 

 same stages of development as those of the bird, which, in the fully deve- 

 loped state, quite resemble them. 



The eggs were incubated by artificial heat, and the hearts of more than 

 fifty embryos, at various stages of development, were examined. The 

 embryos were prepared by immersing them, immediately on their removal 

 from the egg, in strong alcohol. By this the large vessels were obtained 

 distended with blood and hardened. They were afterwards rendered trans- 

 parent by soaking them in strong glycerine, in which they were dissected 



* Kolliker, ' Entwickelungsgeschichte des Meuschen,' pp. 404, 405 (1861). 



t I have left this word untranslated because no single English word exactly ex- 

 presses its meaning. It is obviously the 'bright area of the interior of a transverse 

 section of the vessel held up to the light. iThe boundary of the bright area shows the 

 form of the vascular canal at this point. 



