CHANGES DURING SIXTH AND SEVENTH DAYS. 421 



vascular area. The allantois forms a large vascular patch on 

 the right side, and the embryo is so curved that head and 

 tail nearly meet. The limbs have greatly increased and become 

 bent to form the elbow and knee, and the parts to become bone 

 are invaded by cartUage. Cartilaginous bars also arise in the body 

 walls, the ribs : some of the ventral parts of the ribs unite and 

 form a cartilaginous sternum. The bones of the skuU also 

 arise, and cartilaginous rods in the visceral arches. The face 

 undergoes great alterations, including the closing in of the nasal 

 passages by nasal processes. Great changes also appear in the 

 heart : the ventricles become pointed, and the auricular septum 

 begins to show, the cavity of the auricles separating from the 

 ventricles. The ventricles do not completely divide until after 

 the thirteenth day of incubation. It is not until this day that 

 the cells of the chick become properly differentiated, and from 

 now they rapidly vary in each part, forming the various definite 

 tissues that build up the body. 



Changes during the sixth and seventh days. — On these days 

 the embryo becomes distinctly marked with avian features. 

 On opening the egg we shall note that the body is not so flexed, 

 and also that the neck of the bird is more prominent. Still we 

 may observe the heart hanging out of the body ; but it com- 

 mences to become enclosed by the thoracic walls, supported by 

 the developing cartilaginous ribs. The cerebral hemispheres 

 are quite large. By the seventh day we can see no traces of 

 the visceral clefts, and the mouth and face assume avian char- 

 acters. The amniotic cavity is quite large on the sixth day, 

 being filled with liquor amnii, and gradually increases ; and on 

 the seventh day this amnion moves in a rhythmical manner : 

 by this pulsating movement the embryo is rocked to and fro in 

 the egg. 



Fro7n the eighth day onwards. — ^During the eighth, ninth, and 

 tenth days of incubation rapid growth takes place. The allan- 

 tois is spread out as a sac over most of the yolk and serves as 

 the chief organ of respiration, and the yolk-sac rapidly decreases 



