Ven. IV. 



nch'f 



Fig. i66. — Horizontal Section of a Chick Embryo with about Twenty-eight Segments. 

 Am, Am' ,Ain'\ KvRx\\on. ^c?, Aorta. C«^, Cardiac aorta. Ca*, Coelom. Z).^^?, Dorsal aorta. Z??V«, Dien- 

 cephalon. ^«^^, Endothelial heart, /i^, Cerebral hemisphere. ^/.^, Mid-brain. ]\Idb, Mandible. I\Id, 

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

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

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



291 



