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 days x 5. (After Keibel and Abraham.) 



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 



