EARLIEST STAGES OF EMBRYONIC DEVELOPMENT. 581 
power of filling its lungs with air. The yolk-bag floats within 
the albumen, and always tends to take the highest place, being 
the lighter of the two ; but it is kept nearly to one place by 
two cords (¢, e) termed the chalaze, which seem formed of 
peculiarly viscid albumen, and connect the yolk-bag with the 
lining membrane at the two ends of the shell (d, d). In this 
manner the yolk-bag is always kept at the part of the shell 
where it can most favourably receive the warmth imparted to 
it by the mother ; and the cicatricula or germ-spot (which is 
the mass of cells first developed from the germ-yolk) is made, 
by a similar contrivance, always to rise to the highest point.— 
In the eggs of Fishes there is no additional albumen ; and 
in those of Frogs the albumen is common to the general mass 
of ova, constituting the peculiar “ gelatinous envelope,” which 
forms long necklace-like strings of “spawn,” within which the 
black yolk-bags are disposed at tolerably regular intervals. In 
Mammals, each ovum receives a separate investment of a jelly- 
like substance in its passage along the oviduct into the uterus ; 
and around this there is formed a fibrous membrane termed 
the Chorion, which is destined to take a very important share 
in the subsequent nutrition of the embryo (§ 761). 
756. In the eggs of Frogs, as in those of Mammals, the 
whole of the yolk undergoes the process of segmentation 
already described (§ 736), and takes a share in the formation 
of the “mulberry mass.” But in the eggs of Fishes, Birds, 
and the higher Reptiles, this process is limited to that small 
portion of the yolk which is distinguished as the germ-yolk ; 
and the formation of the germinal membrane takes place after 
- adifferent fashion. The mass of cells that immediately results 
from segmentation, flattens itself out on the surface of the 
yolk, forming the minute semi-opaque whitish spot, which is 
known as the cicatricula, germ-spot, or “tread ;” and by a 
further extension it constitutes the “germinal membrane,” 
which gradually spreads itself over the food-yolk ; at the same 
time dividing itself into two layers, between which a third 
is afterwards interposed. Thus the “germinal membrane,” 
which may be compared to the seed-leaves or cotyledons of 
Plants, forms a sort of temporary stomach round the imass of 
nutriment prepared for the sustenance of the embryo; the 
whole of which nutriment, as will be presently seen, is absorbed 
into the body of the embryo through its instrumentality. 
