340 Sir Everard Home on the changes the egg undergoes 
is to be formed, is originally placed on the surface of the 
yelk ; it is found there before the yelk leaves the ovarium, 
and lies loose upon it, not being enveloped in any capsule. 
The external membrane of the yelk is very thin and deli* 
cate, its surface is studded over with red dots, which disappear 
in its passage along the oviduct. When this is removed, 
there is a second thick and spongy covering under it, in which 
there is a natural aperture ; and the areola, surrounding the 
molecule, is nothing more than the surface of the yelk, that 
is circumscribed by the margin of this aperture. No such 
aperture has been before taken notice of. 
The molecule itself has a granulated appearance ; in the 
centre it is made up of globules part of an inch in 
diameter, surrounded by circles of a mixed substance, con- 
sisting of about two-thirds of the same small globules, and 
one-third of larger oval globules, about ttoo P art °f an inch 
in length, and ^croo P art diameter ; these last, in their 
figure, resemble the red globules of the blood in the bird in 
every respect, excepting their red colour. Besides the glo- 
bules there is some fine oil, which appears in drops when the 
parts are immersed in water. Oval globules and oil are also 
met with in the yelk itself, but in small proportion, and with- 
out colour. 
All these parts, except the red dots on the surface, are met 
with in the yelk on a smaller scale, even six days before it 
is completely formed. 
The ovarial yelk-bag gives way at the middle line, far- 
thest from the insertion of the blood-vessels, and the yelk 
drops out into the mouth of the oviduct. 
The yelk-bag does not immediately close, although it con- 
