Development of the Fourth Day 225 



embryo, and project strongly from the sides 

 of the head. The mesoblast surrounding the 

 brain is increasing in amount, and is begin- 

 ning to show sHght indications of the forma- 



^M 



Fig. 72. — Two stages in the development of the chick 



EMBRYO. A, AT ABOUT THE FIFTH DAY OF INCUBATION ; B, AT 

 ABOUT THE NINTH DAY OF INXUBATION. In A THE AMNION HAS 

 BEEN ALMOST ENTIRELY REMOVED, AND IN B ALL OF THE FCETAL 

 APPENDAGES, EXCEPT A SMALL PART OF THE UMBILICAL STALK, HAVE 



BEEN REMOVED. (After Parker and Haswell, from Duval.) 



all^ allantois. am^ cut edge of amnion. ««, anus, au.ap^ auditory aperture. 

 nu.Sy auditory sac. y*.^?", fore-brain. _/"/, fore-limb, h.br^ hind-brain. h,l^ hind- 

 limb, ht^ heart, hy^ hyoid arch, m.b^ mid-brain. ?««, mandibular arch. «rt, 

 nostril. /, tail, 



tion of the skull. All these changes begin to 

 give the anterior end of the chick the appear- 

 ance of a distinct head. The fronto-nasal 

 process (page 195) begins to show promi- 

 nently as an outgrowth from the fore part 



