EMBRYO WITH TWENTY-EIGHT SEGMENTS. 



215 



Ven.IV 



nch' 



FIG. 164. HORIZONTAL SECTION OF A CHICK EMBRYO WITH ABOUT TWENTY-EIGHT SEGMENTS. 

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

 cephalon. Endo, Endothelial heart. H, Cerebral hemisphere. M.b, Mid-brain. Md, Medullary tube. 

 Mdb, Mandible, m.ht, Muscular heart, nch, nch', nch", Notochord. Op, Optic vesicle. Ph, Pharynx. 

 Seg, Segment. Seg.z, Segmental zone of mesoderm. Som, Somatopleure. Sp.c, Spinal conl. Yen, Yen- 

 tricle. Yen. IV, Fourth ventricle or cavity of the hind-brain. X 30 diams. 



