DEVELOPMENT OF THE CHICK. 683 
with which it is already invested ; on the other hand, segmentation goes 
on rapidly in the formative area. : 
The fully formed and laid egg is surrounded by a firm porous shell of 
carbonate of lime, and beneath this there is a double shell membrane, 
the two layers of which are separated at the broad end of the shell to 
form an air-chamber. This chamber grows larger as development pro- 
ceeds, and is of some importance in connection with respiration, as an 
intermediate region between the embryo and the external medium. 
Beneath the shell membranes lies the albumen, or ‘‘ white of egg,” 
which is secreted by the thin-walled region of the oviduct ; in it lie two 
spirally-twisted cords or chalazze, produced by the rotation of the egg in 
the oviduct. Within the enveloping 
albumen lies the ovum proper, with 
its enormous mass of yolk. The 
yolk is not homogeneous, but con- 
sists of two substances, known re- 
spectively as white and yellow yolk. 
The white yolk forms a central flask- 
shaped mass, and occurs also as thin 
concentric layers in the yellow yolk. 
The minimum temperature at 
which a hen’s egg will develop nor- 
mally is 28°C. If the temperature , : . 
fall below this, development stops.  F16. 379.—Diagrammatic section 
In early stages the interruption may of egg.—After Allen Thomson. 
last for days without fatal’ results, g.v., Position of germinal vesicle; 
though always with a tendency to #¢ Rie ener ccm yolk a 
induce subsequent abnormalities. rite”) ch, chalaze 
Towards the end of incubation more oy 
than a day’s cooling is usually quite fatai. 
On the upper surface of the yolk, in whatever position the egg be 
held, lies the segmented blastoderm, whose exact origin we must con- 
sider more precisely. 
As we have seen, yolk is to be regarded as an inert and passive sub- 
stance. In the hen’s egg we have an increased specialisation along 
the line indicated by the egg of the frog. For there is a small patch 
of formative protoplasm at one pole, and a large aggregate of yolk 
composing the remainder of the egg. In consequence, the activity of 
3. Diagrammatic surface view. .f., Area pellucida; a.o, area 
opaca; #.p., neural groove ; Z.s., primitive streak ; 47., meso- 
blast spreading over yolk. 
4. Diagrammatic surface view at later stage. a.., Area pellucida ; 
@.0., area opaca; m.s., mesoblast segments; #.s., primitive 
streak. The dark border shows the spreading of the mesoblast 
over the yolk. 
s. Cross-section. s.c.,Spinal cord ; s.g., rudiment of spinal ganglia ; 
N., notochord; #2.g., mesoblastic plates; A., aorta; Am., 
amnion fold; ¢., coelom or pleuro-peritoneal cavity. 
6. Embryo. Cé., Cerebellum; #., ear; H., heart; ., fore-limb ; 
4.1, hind-limb; y.s., stalk of cut-off yolk-sac; AZ, allantois ; 
E., eye; C., cerebrum. On the dorsal surface the mesoblastic 
somites are indicated. 
