during incubation in the common fowl. 343 
circumscribed. This I shall call the outer areola. In the 
space between these two areolas, there were distinct dots of an 
oily matter. This extension of membrane to the outer areola 
lies under the inner membrane of the yelk, and can readily 
be removed entire. Plate XXXIII. fig. 4, 5, 6. 
In 12 hours, the rudiments of the brain were more distinct, 
as well as of the spinal marrow. These parts were placed upon 
a black ground in vinegar, and hardened ; the upper end 
showed the tuberculum annulare of the brain, from which 
passed down two semi-transparent lines resembling an ap- 
pearance peculiar to the spinal marrow of the bird. Plate 
XXXIII. fig. 7, 8, 9. 
In 1 6 hours, there was a farther advance in the structure of 
all these parts. Plate XXXIV. fig. 1, 2, 3. 
In 24 hours, a still greater increase. Plate XXXIV. fig. 
6 - 
In 3 6 hours, the head was turned to the left side. The 
cerebrum and cerebellum appeared to be distinct bodies; the 
iris was seen through the pupil of the eye. The interver- 
tebral nerves were nearly completely formed ; those nearest 
the head the most distinct. A portion of the heart was seen. 
At this period, under the inner areola, apparently at the 
termination of the spinal marrow, a vesicle had begun to 
protrude. In some eggs it is seen earlier than in others, and 
has been observed before the heart had become visible. Plate 
XXXIV. fig. 7, 8, 9. 
In 2 days 12 hours, the spinal marrow was found to have its 
posterior part enclosed ; the auricles and ventricles of the 
heart were seen, the auricles filled with red blood. An 
arterial trunk from the left ventricle gave off two large 
