STUDY OF SECTIONS OF EMBRYO OF 24 MM. 335 



almost as great as that occupied by the rest of the body. The great size of the 

 head at this stage is characteristic. Attention is especially directed to the sharp 

 angle which the medulla oblongata, Md.ob, makes with the spinal cord, Sp.c, and 

 to the very great bend formed by the floor of the mid-brain, Ar.hab, in conse- 

 quence of which the floor of the hind-brain above the medulla oblongata and the 

 floor of the fore-brain are brought quite close together and run in almost parallel 

 directions. The cavity of the brain is very large. Its walls in the median plane 

 are, for the most part, thin. From the roof of the diencephalon, Dien, there runs 

 off a small evagination, Ephys, a shallow pocket or diverticulum of the medullary 

 wall. It is the anlage of the epiphysis or pineal organ of the adult. It is an 

 important landmark in the topography of the brain, for its position is always at 

 the extreme posterior limit of the fore-brain. In the wall of the mid-brain, behind 

 the epiphysis, for some distance the ectoglia shows considerable thickening and 

 contains a very large number of nerve-fibers running transversely. They constitute 

 the posterior 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 another 

 tract of nerve-fibers which is very small and is known as the superior commissure. 

 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 epiphy- 

 sis the dorsal roof of the diencephalon forms a broad arch which descends in 

 the figure vertically until it ends in a small inward projection, Fix, of the brain- 

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

 forming the lamina lerminalis. 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 infundibular gland, which 

 bends over so as to lie close to the dorsal side of the hypophysis, Hyp. The 

 hypophysis, which in earlier stages appears clearly as an evagination of the oral 



FIG. 224. 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 aorta. 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. CW, Cerebellum. Coe, Coelom. Diaph, Diaphragm. Dien, Dience- 

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

 Hyp, Hypophysis. In, Intestine. Int.v, Anlage of intervertebral ligament. La, Lateral wall of larynx. Li, 

 Liver. Lu, Lung. M.b, Mid-brain. Md.ob, Medulla oblongata. Nch, Notochord. (E, (Esophagus. 

 Pen, Penis. Ph, Pharynx. Plx, Choroid plexus of fore-brain. Plx.IV, Choroid plexus of hind-brain. 

 Sept, Cartilaginous nasal septum. Sp.c, Spinal cord. Sp.ren, Suprarenal capsule. St, Stomach. T',T", 

 Tail. Te, Testis. Ton, Anterior portion of tongue. Umb, Umbilical cord. Ve, Cardinal vein. Ven, 

 Ventricle of the heart. Ven. I V, Fourth ventricle, or cavity of the hind-brain. Vert, Vertebra. W.b, Wolman 

 body. X 8 diams. 



