SO MEMOIR OF BARON HALLER. 
viscera is placed exterior to the body of the chick, 
and is connected only by a canal of communication. 
During the latter stages of incubation, and especially 
in the hatched chicken, things are quite altered. 
The internal viscera now have become large and 
visible, the canal of communication and the yolk 
have faded and disappeared, and the chick has 
nothing pertaining to it external to itself. Again, 
the dorsal aorta of the chick, before it is hatched, 
appears to be a common trunk with three branches, 
two of which belong to the pulmonary artery, and 
the third to the left ventricle of the heart ; but 
after it is hatched, the aorta is only a simple artery, 
proceeding from the left ventricle, and having no 
connexion with the pulmonary vessels. Once more, 
the chick of the first day is scarcely more than a 
head with a slender thread, which is the spinal 
column ; when twenty-two days old, the extremities 
and viscera have been elaborated out of this almost 
invisible appendage, and the head in its turn has 
become an appendix. 
Relative change of place is another instrument 
employed by nature. Of this we see an example 
in the yolk and intestines. Both these are external 
to the chick, almost to the termination of incuba- 
tion, and the embryo being appears to have two 
bodies communicating together, the one consisting 
of the head, extremities, and internal cavities, and 
the other of the yolk, the umbilical membrane, and 
the intestines, all parts of the chick, and yet de- 
tached from it. The membrane fades and disap- 
