xm PRACTICAL DIRECTIONS 617 



from the fore-brain ; c, the auditory pits, one on either side 

 of the hind-brain region ; d, the increase in number of the 

 mesodermal segments ; e, the heart and main vitelline veins 

 (Figs. 518, A, and 159, A). 



5. Third day. Note a, the further extension of the area 

 vasculosa and its blood-vessels and the folding off of the 

 embryo from the rest of the blastoderm investing the yolk ; 

 b, the amnion ; c, the head, which is proportionately very 

 large, has become twisted over so that its right side is now 

 uppermost, and has undergone flexure, the prominent mid- 

 brain being now at the anterior end ; observe the cerebral 

 hemispheres and the cerebellum in addition to the other parts 

 of the brain ; d, the visceral arches and clefts ; e, the 

 olfactory pits, optic cups and lens, and closed auditory 

 vesicles ; /, the mesodermal segments ; g, the S-shaped heart, 

 arterial arches, dorsal aorta, and vitelline veins (Figs. 158, B, 

 and 159, B). 



6. Fourth to sixth days. Note a, the advances in the 

 parts mentioned above, including the further extension of 

 the area vasculosa over the yolk, the narrowing of the yolk- 

 stalk, the flexure of the whole embryo, and the amnion ; 

 b, the budding fore- and hind-limbs ; c, the allantois, at first 

 a small sac extending from the ventral side of the body and 

 gradually increasing in size (Figs. 158, C, and 159, C). By 

 the sixth day, the embryo can be recognised as that of a 

 bird from the characters of the fore-limbs (wings), hind- 

 limbs, head, etc. : feather-rudiments cannot be recognised 

 until the ninth day. 



7. A few further stages should be examined (compare Fig. 

 159, D), especially that just before hatching, at the end of 

 the third week of incubation, when it will be found that the 

 albumen has become absorbed, and the very vascular allan- 

 tois will be seen all round the egg directly under the shell, its 

 stalk entering the body at the umbilicus ; the remains of 

 the yolk-sac have been withdrawn into the body at the 

 same point. Note also the neb or caruncle on the beak, 

 used for breaking the egg-shell at hatching. 



II. Transverse sections. 



1. Unincubated egg. 1 Note the ectoderm and the lower 

 layer cells (Fig. 153, F). 



2. First day of incubation. Less difficulty will be experi- 

 enced if you select embryos towards the end of the first day 

 (20-24 hours). Distinguish between the ectoderm, endoderm, 

 and mesoderm. Note a, the formation of the medullary 

 groove and folds ; b, the primitive streak and groove ; c, the 

 notochord (Figs. 153, G, and 155). 



1 See note about early stages on p. 616. 



