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 slight indications of the forma- 



7t.l 



FlG. 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 INCUBATION. IN A THE AMNION HAS 

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

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



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



a//, allantois. am, cut edge of amnion. an, anus, au.ap, auditory aperture. 

 au.s, auditory sac. f.br, fore-brain, f I, fore-limb, h.br, hind-brain. Jt.l, hind- 

 limb, ht, heart, hy, hyoid arch. /., mid-brain, mn, mandibular arch. a, 

 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 



