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- 



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 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. an, anus, au.afi, auditory aperture. 

 au.s, auditory sac. f.br, fore-brain. _/"/, fore-limb, h.br, hind-brain, h.l, hind- 

 limb, ht, heart, hy, hyoid arch, m.b, mid-brain, mn, mandibular arch. na, 

 nostril. t, 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 

 15 



