VALVES IN THE HEAET OF THE CHICK. 
389 
being readily altered to a fresh position where it was necessary that the same object 
should be seen from different points of view. The observations were made on more 
than fifty embryos in various stages of development, and the most satisfactory dissections 
were converted into permanent preparations. The preliminary experiments on artificial 
incubation, as well as on the best method of preparing the embryos, consumed much 
time. Much time and pains were also spent in conducting the incubating-process, and 
before I had gained experience the incubation of a number of eggs for several days often 
led to no result. 
The account of my own observations will be best prefaced by quoting the statements 
of Rathke and Kolliker with reference to the development of this part of the heart ; 
Kolliker’s observations are the latest I have been able to discover. 
Rathke* says, “The anterior chamber of the heart remains single in fishes, and in 
them developes itself into the so-called bulbus aortse (Herz-Zwiebel). In the Batrachia it 
lengthens itself, and forms the trunk for all the arteries of the body. This, however, 
does not remain so smooth and simple internally as on the exterior ; but the wall of the 
anterior chamber of the heart while it lengthens sends inwards two longitudinal ridges 
situated opposite to one another, so that the cavity of the arterial trunk which arises 
from it becomes very incompletely divided into two lateral halves. The same develop- 
mental process is also remarked in Birds and Mammalia ; in them, however, the two 
ridges become higher, then grow together throughout their whole length, and completely 
divide the hollow of the chamber into two lateral halves. When this has taken place, 
a division results by a resorption through the middle of the wall and newly formed 
septum of the anterior chamber of the heart ; and this now becomes divided into two 
canals, of which one represents the commencement of the aorta, the other the commence- 
ment of the pulmonary artery. In the Snake, and probably also in other reptiles, three 
partitions arise on the inside of the anterior chamber of the heart, the free borders of 
which ultimately grow together, and so the chamber gets divided into three canals lying 
beside one another, of which one represents the commencement of the pulmonary artery, 
the two others the commencement of the two roots of the aorta.” Rathke’s description 
therefore relates solely to the division of the truncus arteriosus. With reference to the 
development of the semilunar valves he is silent. 
KoLLiKER’sf account of the division of the truncus arteriosus is as follows : — “ Simul- 
taneously with the formation of the septum of the ventricles proceeds also the division 
of the truncus arteriosus into the pulmonary artery and permanent aorta , which, though 
apparently only a continuation of the process which takes place in the separation of the 
ventricles, is yet to be distinguished from it. Thus while in the ventricle^ it is the mus- 
cular structure of the heart which grows out and ultimately developes itself into a com- 
plete septum, in the primitive aorta it is the vascular wall, having more the structure of 
connective tissue, that effects the separation. Therefore the division of the truncus arte- 
* Entwickelung der Wirbelthiere, S. 82, pp. 187-188 (1861). 
f Entwickclungsgeschichte des Menseben, pp. 404, 405 (1861).. 
