EMBRYO WITH EIGHT SEGMENTS. 



181 



cerebral vesicle, fore-brain or prosencephalon. The second cerebral vesicle, mid-brain 

 or mesencephalon, Br.2, comprises the part of the medullary tube immediately 

 behind the optic vesicles and includes a little more of the length of the tube 

 than the vesicles. In older embryos (14-20 segments) it becomes clearly marked 

 off by constrictions from both the fore- and hind-brain. The third cerebral 



G. Pro. am. Np. Op. V. a. 



Br. 



Br.3 



Fov 



Sp. c. 



Sg. z. 



V. t. 



Pr. s. 



FIG. 131. CHICK WITH EIGHT FULLY FORMED SEGMENTS, STAINED WITH ALUM COCHINEAL AND MOUNTED IN 



DAMAR. 



a, Outline of the free portion of the head. A.c.v, Amnio-cardiac vesicle. A.p', A.p", Area pellucida. Bl.is, 

 Blood-island. Br.2, Second cerebral vesicle. Br.$, Third cerebral vesicle. Fov, Fovea cardiaca. G, 

 Post-optic ganglionic crest. Md, Medullary groove. Nch, Notochord. Np, Anterior neuropore. Op.V, 

 Optic vesicle. Ph, Outline of pharynx. Pro., am, Pro-amniotic area. Pr.s, Primitive streak. Sg.z, Segmental 

 zone or band of mesoderm, from which new segments arise. Som.^, Third somite. Sp.c, Spinal cord. V.om, 

 Vena omphalo-mesaraica. V.t, Vena terminalis. X 15 diams. 



vesicle, hind-brain or rhombencephalon, Br.T,, is as long as the other two and has 

 a tapering form; it becomes the cerebellum, pons, and. me*dulla oblongata of 

 the adult. The cavity of the brain is wide, but in the region of the hind- 

 brain it tapers caudad and so passes over into the narrow cavity of the spinal 



