EMBRYO OF 24 MM. 265 



some distance the ectoglia shows considerable thickening and contains a very 

 large number of nerve-fibers running transversely. They constitute the poste- 

 rior commissure, which morphologically belongs to the mid-brain. In later 

 stages the opening of the epiphysis and the anterior boundary of the posterior 

 commissure are separated by a narrow band of ependyma. Immediately in 

 front of the epiphysis, close to the external surface of the medullary wall, is an- 

 other tract of nerve-fibers which is very small and is known as the superior com- 

 missure. The superior commissure develops much later than the posterior, and 

 is much smaller in size. The two commissures are found in vertebrates of all 

 classes and are exceedingly constant anatomical features of the brain. Anterior to 

 the epiphysis the dorsal roof of the diencephalon forms a broad arch which de- 

 scends in the figure vertically until it ends in a small inward projection, Plx, of the 

 brain-wall, the anlage of the choroid plexus. Below this point the brain-wall is 

 continued, forming the lamina terminalis. It then makes a bend almost at right 

 angles and runs in a horizontal direction toward the dorsal side of the embryo. 

 This portion of the brain- wall shows a considerable thickening, the optic chiasma. 

 Behind the optic chiasma the brain-wall forms a short evagination, the infundi- 

 bular gland, which bends over so as to lie close to the dorsal side of the hypophy- 

 sis, Hyp. The hypophysis, which in earlier stages appears clearly as an evagina- 

 tion of the oral epithelium (Fig. 123), is now entirely separated from the mouth, 

 and is an epithelial vesicle with an irregular cavity. The epithelium has sent out, 

 especially on its anterior side, a number of solid outgrowths. The infundibular 

 gland and hypophysis constitute the pituitary body of the adult. They are sur- 

 rounded by loose mesenchymal tissue. The sella turcica, in which the pituitary 

 body of the adult is lodged, is already marked out, because the chondrification, 

 which is to form the sphenoidal cartilages, has already begun about these struc- 

 tures. The sphenoidal cartilage is continuous, on the one hand, with that of the 

 nasal septum, Sept; and, on the other, with that of the vertebral column, Vert. 



Fig. 151. — Pig, 24.0 mm. Sagittal Series 63, Section 30. ^ 

 A, Arachnoid space, in this specimen containing extravasated blood. A.Ao, Arch of the main aorta. All.ar, 

 Allantoic artery. Ao, Dorsal aorla. Ao.D, End of dorsal aorta. Ar.hab, Habenular arch (floor of mid- 

 brain). A.vert, Vertebral artery joining its mate to form the basilar artery. Bro, Main bronchus of lung. 

 bro, Branch bronchus within the lung. Cbl, Cerebellum. Car, Crelom. Diap/i, Diaphragm. Dien, Dien- 

 cephalon. Duo, Duodenum. Epen, Ependymal roof of hind-brain. Ephys, Epiphysis. G, Spinal 

 ganglion. Hyp, Hypophysis. In, Intestine. Inl.v, Anlage of intervertebral ligament. La, Lateral wall 

 of larynx. Li, Liver. Lit, Lung. M.b, Mid-brain. Md.ob, Medulla oblongata. Nch , Notochord. 

 (E, Oesophagus. Pin, Penis. PA, Pharynx. Plx, Choroid plexus of fore-brain. Plx. IV, Choroid plexus 

 of hind-brain. Stpt, Cartilaginous nasal septum. Sp.c, Spinal cord. Sp.ren, Suprarenal capsule. St, 

 Stomach. T',T", Tail. Tt, Testis. Ton, Anterior portion of tongue. Umb, Umbilical cord. Ve, Car- 

 dinal vein. Ven, Ventricle of the heart. Vtn.IV, Fourth ventricle, or cavity of the hind-brain. Vert, 

 Vertebra, ll'.b, Wolffian body. X 8 diams. 



