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EMBRYOLOGY. 
127 
egg in a slightly more advanced stage be now examined 
from a horizontal point of view, there will be seen a light 
oval space, the area pellucida, surrounded by a dark space, 
the area opaca (Fig. 166); within the area pellucida will 
. be noticed an oval body, the Primitive trace, so called from 
indicating the position of the embryo, the furrow in the 
Primitive trace being known as the Primitive groove. A 
little later the Primitive trace and area pellucida become 
' guitar-shaped (Fig. 167), and if a longitudinal section of 
the egg be examined (Fig. 168) it will be seen to consist 
of the External and Internal blastodermic membranes, and 
a third membrane lying between these two. The partial 
fusion of these membranes makes the Primitive trace. 
While these three membranes are consolidating into the 
Primitive trace, the Middle membrane splits into two 
layers: the Upper terminates in the External blastodermic 
membrane, the Lower grows gradually around the In- 
ternal blastodermic membrane, finally inclosing it. The 
embryo at this period is a guitar-shaped body (Fig. E77), 
consisting simply of three membranes lying over one 
another, narrowly bound together. The question may be 
asked by some of our readers, What relation does so 
minute a structure as the egg of a mammal bear to that of 
abird? Does the development of a rabbit resemble that 
of the chick? The egg of a chicken (Fig. 174), as all the 
world knows, is composed of a shell inclosing a semi-liquid 
substance, in which is suspended a yolk. If a freshly-laid 
egg be carefully examined, however, supposing the condi- 
tions to have been favorable to development, there will be 
found lying on the top of the yolk a delicate sheath (Fig. 
174, 6), which is composed of two membranes; while the 
yolk itself, if laid open, exhibits in its interior a whitish 
body (Fig. 174, a), which, narrowing into a thread, runs 
upwards towards the membrane composing the sheath. 
This whitish substance is called the white yolk, as distin- 
