390 
DE. M. TONGE ON THE DEVELOPMENT OF THE SEMILHNAE 
riosus cannot be described as if it were formed by an inward growth of the ventricular 
septum, as also comes out in the clearest manner from the fact that in certain creatures 
the aorta divides at a time when the ventricle is still single. This is the case in the 
Viper, according to Rathke, in which, at the time when the truncus arteriosus divides 
into three vessels, the ventricle still possesses no trace of a septum. Therefore, as Rathke 
correctly observes, the cause of the division of the primitive aorta into two canals cannot 
with Von Baer be sought in certain peculiarities of the circulation, in a particular direc- 
tion of the current of blood, but it depends solely and alone on particular developmental 
phenomena of the arterial wall. Now, concerning the peculiarities in man, I have in the 
fourth week found the truncus arteriosus still completely single, and with round lumen*. 
Transverse sections examined under the microscope already show clearly three tunics, — 
a thin tough adventitia , a strong clear media , and an inner cellular layer as intima. In 
the fifth week likewise the artery was still single, but its lumen was now flattened out 
transversely and slit-shaped. In the seventh and eighth week I found the vessel already 
completely doubled, and I was not here able to make out the intermediate stages, and 
to follow the gradual formation of the septum. I was more fortunate with calf embryos 
of T"—S"' long ; and I here first found an aorta with a lumen like a figure of 8, or, in 
other words, with two slight ridges in the interior, which resulted from growths of the 
tunica media, and, secondly, others which contained two lumina within a common adven- 
titia ;, each of which possessed its own separate tunica intima , but connected tunicas mediae. 
In consequence of this it cannot well be doubted that the division of the truncus arte- 
riosus essentially comes to pass by a growth of its middle coat, followed at a later period 
by that of the adventitia , which is, however, in Man effected very early, since even at 
the eighth week both the large arteries possess all their proper coats.” 
With respect to the development of the semilunar valves, he says, “ Simultaneously 
with the division the semilunar valves also become developed, and I saw them already 
present in both arteries in an embryo of the seventh week. They are, however, at first 
nothing but horizontally 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.” 
I now pass on to my own observations. They were made only on the embryo of the 
fowl, as I have had no opportunity of investigating that of Man or any other mamma- 
lian animal. But from the great likeness between the hearts of Birds, Mammalia, and 
Man at different stages 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 developed state quite resemble 
them. 
As is well known, the heart of Birds and Mammalia in its earliest condition is a simple 
* I have left this word untranslated because no single English word exactly expresses its meaning. It is 
obviously the bright area of the interior of a transverse section of the vessel held up to the light. This bright 
Area being bounded by the inner edge of the transverse section, shows the form of the vascular canal at this point. 
