476 



DEVELOPMENT OF THE CHICK. 



definite layer and is raised up from the yolk over that portion 

 of the head which is folded off from the egg ; while elsewhere 



Fia. 86. A Chick Embryo at the thirty-sixth hour of incubation ; seen 



from the dorsal surface, x 20. 

 Flo. 87. A median longitudinal, or sagittal, section of a Chick Embryo 



at the thirty-sixth hour of incubation, x 20. 



AN, head fold of the amnion. BF, fore-brain. BH, hind-brain. BM, mid- 

 brain. BO, optic vesicle. CH, notochord. CP, pericardial cavity. El, audi- 

 tory pit. GF, fore-gut, or anterior portion of the mesenteron. H, hypoblast. 

 MS, mesoblastic somite or protovertebra. NS, spinal cord. NT, neurenteric 

 canal. PS, primitive streak. RV, ventricular portion of tl e heart. SO, so- 

 matopleure. SP, splanchnopleure. TA, allantois. VV, vitelline veins. 



