EMBRYO AND EMBRYONIC MEMBRANES 213 
portion of the first visceral cleft alone is visible as the external 
auditory meatus; the other visceral arches and clefts have prac- 
tically disappeared, excepting the mandibular arch, forming the 
lower jaw. The abdominal viscera begin to protrude. In the 
next stage, Fig. 123 (eight days), the contours of the body are 
decidedly bird-like; feather germs have appeared in definite 
Fig. 123.— Embryo of 8 daysx 5. (After Keibel and Abrabam.) 
tracts; the fore-limbs are wing-like. The contours of the head 
are much smoother, and determined more by the development 
of the facial region and skull than by the brain. The protuber- 
ance of the ventral surface caused by the viscera is strongly 
marked. Fig. 124 finally shows a ten-day embryo. 
Head. The embryonic development of the head depends on 
the changes in three important classes of organs, together with 
