SAGITTAL SECTIONS OF EMBRYO OF 12 MM. 293 



ing of the brain-wall in later stages furnishes the passage for the fibers of the optic 

 nerve, and is, therefore, the anlage of the optic chiasma. Between the infundibular 

 gland and the optic chiasma extends the post-optic lamina, L.p.o. On the opposite 

 side of the chiasma follows the lamina terminalis, which leads us forward to the 

 wall of the hemispheres, H. Underneath the hind-brain extends the large basilar 

 artery, A.bas; at its posterior end, A.bas.p, the basilar artery is joined by the two 

 vertebral arteries from the fusion of which it is really produced. Underneath the 

 fore-brain we have the opening of the mouth, Or, from the dorsal side of which 

 springs the elongated evagination of the hypophysis. The oral cavity runs into the 

 pharynx, the floor of which is formed in part by the anlage of the tongue, Ton, and 

 of the epiglottis, Epgl, a rounded eminence . very different in shape at this stage 

 from the adult epiglottis. The pharynx can be followed along until it passes over 

 into the oesophagus, (E, which, however, is not well shown, as the section passes 

 through it away from the true median plane. Between the oesophagus and the 

 anlage of the epiglottis is a mound of tissue, La, which represents the lateral wall 

 of the developing larynx. The mound is separated from the anlage of the epiglot- 

 tis by a deep notch. In the median plane the mound is filled with entoderm 

 which forms a wide plate through which there is only a narrow opening leading 

 down into the trachea. Finally, we see from the base of the mandible the somato- 

 pleure, Som, extending off to form the boundary of the pericardial chamber. The 

 figure also includes a presentation of the inferior maxillary vein, V. mx.i, and of the 

 thyroid gland, Thyr, which immediately overlies the main trunk of the ventral 

 aorta. This aorta gives off on either side of the pharynx three principal branches, 

 of which the smallest is the base of the carotid and corresponds to the third aortic 

 . arch. The second and third branches are much larger and correspond to the third 

 and fourth aortic arches. The pulmonary aorta, P.Ao, is already separated from 

 the main aorta of the body. 



Sagittal Section of the Head through the Principal Ganglia (Fig. 202). The 

 section is to one side of the median plane. It exhibits the optic nerve, the tri- 

 geminal, acustico-facial, petrosal, jugular, and nodosal ganglia; but, on the other 

 hand, exhibits little of the brain, there being only a shaving from the lateral wall 

 of the fore-brain, H, and a section of the widest part of the hind-brain which 

 shows the cavity or lateral recess, R.L, of the fourth ventricle. The auditory vesicle 

 is cut, Ot. It is formed by a layer of epithelium derived from the ectoderm, 

 although now not connected with the overlying part of the epidermis by the in- 

 vagination of which the octocyst is developed. It shows a narrow, upward pro- 

 longation, the anlage of the ductus endolymphaticus (compare Fig. 42). The 

 epithelial otocyst lies in a line with the great cephalic ganglia and occupies its 

 invariable and permanent position behind the acustico-facial ganglion, Ac.F, and in 

 front of the glosso-pharyngeal, G.petr. The position of the otocyst makes it an 

 invaluable landmark in the stud rtions of the head. Only the lateral no 



of the pharynx. Ph, appears. It forms a \vicl< it slit-like diverticulum. 



