GENERAL ACCOUNT. 



225 



The first trace of the embryo appears in the centre of the 

 area pellucida, about the twentieth hour of incubation ; the 

 formation of the embryo consisting essentially in a process of 

 folding off, or constriction, of the central part of the area pellu- 

 cida from the rest of the yolk. By the middle of the second 

 day the embryo (Fig. 98) measures about 5 mm. in length, and 

 has acquired definite shape ; the brain, spinal cord, heart, and 

 other organs being already established (cf. Figs. Ill and 112). 



YS 



\ 



sv 



FIG. 100. The Hen's Egg at the end of the fifth day of incubation, seen 

 from the side. The embrj-o, which naturally lies with its left side on 

 the yolk-sac and its right side towards the egg-shell, has been lifted up, 

 in order to show its shape more clearly. The structure of the embryo at 

 this stage is shown on a larger scale in Figs. 115 and 123. x |. 



AN, inner or 'true' amnion. AV, outer margin of area vasculosa. AZ, outer or 

 'false' amnion, together with the vitelliiie membrane. EM, embryo. SH, egg-shell. 

 SM, shell membrane. SV, air chamber. TA, allantois. YS, yolk-sac. 



The embryo, at this stage, lies with its dorsal surface towards 

 the shell, and its ventral surface towards the yolk. The axis of 

 the body is straight, and is usually directed across the axis of 

 the egg ; the head end of the embryo, in the majority of cases, 

 pointing away from the observer if the egg is placed before him 

 with the broader end to his left. There are, however, great 

 variations in this respect, and the axis of the embryo may form 

 almost any angle with that of the egg. 



Q 



