Ven. IV 



nek". 



Fig. 166,-Hoeizontal Section of a Chick Embryo with about Twenty-eight Segments. 

 Am, Am', Am", Amnion. Ao, Aorta. C.ao, Cardiac aorta. Cm, Coelom. Z)^ Dorsal aorta. £««, Dien- 

 cephalon. £«<&, Endothelial heart. //, Cerebral hemisphere. AT. *, Mid-brain. Mdb, Mandible 1/,/ 

 Medullary tube. «.*/, Muscular heart. «*5, «*4', »*i", Notochord. 0j>, Optic vesicle. Ph, Pharynx.' 

 5y, Segment.^ Stg.z, Segmental zone of mesoderm. Som, Somatopleure. Sp.c, Spinal cord. Ven, Ven- 

 Fourth ventricle or cavity of the hind-brain. X 3° diams. 

 291 



tricle. Ven. IV, 



