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. Feather 

 germs have appeared in definite tracts. In the next stage, Fig. 

 123 (eight days), the contours of the body are decidedly bird- 



FiG. 123. — Embryo of 8 days x 5. (After Keibel and Abraham.) 



like; 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 



