CHAPTER III 
OUTLINE OF DEVELOPMENT, ORIENTATION, CHRO- 
NOLOGY 
THE preceding chapters have traced the development up to 
the time of laying. The formation of the germ-layers has begun; 
and the stage of development is fairly definite, though not abso- 
lutely constant. When the egg cools, after laying, the develop- 
ment ceases, but is renewed when the temperature is raised to 
the required degree by incubation. 
On the surface of the yolk is a whitish dise about 4 mm. in 
diameter, known as the blastoderm. Edwards gives the average 
diameter of the unincubated blastoderm (59 eggs) as 4.41 mm., 
of the area pellucida (50 eggs) as 2.51 mm. The central part 
of the blastoderm is more transparent and is hence known as 
the area pellucida; beneath it is the subgerminal cavity. The 
less transparent periphery is known as the area opaca. In the 
course of development the embryo and the embryonic mem- 
branes, which serve for the protection, respiration, and nutrition 
of the embryo, arise from the blastoderm. 
The embryo proper arises within the area pellucida, which 
becomes pear-shaped as the embryo forms; the remainder of the 
blastoderm beyond the embryo is extra-embryonic. From it 
arise the embryonic membranes known as the amnion, chorion, 
and yolk-sac. The allantois (Fig. 33 B) arises as an outgrowth 
from the hind-gut of the embryo, and spreads within the extra- 
embryonic body-cavity; it thus becomes an extra-embryonic 
membrane secondarily. The growth of the embryo and of the 
extra-embryonic blastoderm are distinct, though interdependent, 
processes going on at the same time. 
During the first four days of development the blastoderm 
spreads very rapidly (Figs. 32 and 33). ‘Thus on the fourth day 
(Fig. 33 A) the greater portion of the yolk is already covered. 
Thereafter the overgrowth of the yolk proceeds much more slowly 
(ef. Fig. 33 B). In the opaque area there arise,as concentric zones, 
the area vasculosa distinguished by its blood-vessels and the area 
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