M.b. 



Yen. IV. 



Ao. 



Am". 



Scg.z. 



Sp.c. 



FIG. 166. HORIZONTAL SECTION OF A CHICK. EMBRYO WITH ABOUT TWENTY-EIGHT SEGMENTS. 

 Am, Am', Am", Amnion. Ao, Aorta. C.ao, Cardiac aorta. Cce, Coelom. D.Ao, Dorsal aorta. Dien, Dien- 

 cephalon. Endo, Endothelial heart. H, Cerebral hemisphere. M.b, Mid-brain. Mdb, Mandible. Jl/d, 

 Medullary tube, m.ht, Muscular heart, nch, nch' , nth", Notochord. Op, Optic vesicle. Ph, Pharynx. 

 Seg, Segment. Seg.z, Segmental zone of mesoderm. Som, Somatopleure. Sp.c, Spinal cord. J'en, Ven- 

 tricle. Ven. IV, Fourth ventricle or cavity of the hind-brain. X 3 diams. 



291 



