' FRONTAL SECTIONS OF EMBRYO OF 12 MM. 



299 



epen 



tinctly marked bundle of nerve-fibers which arise from neuroblasts of the ventral 

 zone, traverse that zone almost horizontally, then bend downward and pass out 

 from the brain-wall, appearing as the lateral root of the facial nerve. The root 

 runs first toward and then past the geniculate ganglion. The cardinal vein origi- 

 nally was inside the ganglia; by island formation it has migrated outside the ganglia, 

 forming the lateral vein, Jug. In the mandible below the pharynx appear two 

 nerves. Of these, the upper is the hypo- 

 glossal, N.I 2, which lies near the angle 

 formed by the junction of the first gill-cleft 

 with the pharynx. The lower of the two 

 nerves, Mx.i, is the inferior maxillary. 



Section through the Otocyst (Fig. 205). 

 The figure is from a section not far from 

 the last. The hind-brain has narrowed 

 considerably; its thickened floor, Md.obl, the 

 anlage of the medulla oblongata, rises 

 steeply from the median line. Its ependymal 

 roof, epen, is less expanded than in figures Fac.m. 

 204 and 205. It forms a sharp angle in 

 the * dorsal median line. The median ven- 

 tral fissure between the two sides of the 

 medulla is deeper than farther forward. 

 The pharynx, Ph, is wide and has expanded 

 laterally into the common beginning of the 

 first and second gill-pouches. Between the FIG. 205. PIG, 12.0 MM. FRONTAL SERIES '6, 

 pharynx and the raphe the basilar artery, SECTION 380. 



A.bas, his been CUt transversely. Below ^, Basilar artery. Coch, Cochlea. Z>. e ,Ductus 



endolymphaticus. epen, Ependyma. Fac.m, 



D.e. 



S.c. 



Jug. 



Coch. 



Ph. 



Motor division of the facial nerve. Jug, Vena 

 lateralis capitis. Md.obl, Medulla oblongata. 

 Ph, Pharynx, raph, Median raphe of the 

 medulla oblongata. S.c, Anlage of the semi- 

 circular canals. Ve, Vein. X 22 diams. 



it and near the pharynx is the small 

 notochord, which, however, can b.e clearly 

 recognized only with the higher power, 

 and is, therefore, not represented in this or 

 the preceding figure. The otocyst is a large 

 epithelial vesicle with three well-marked divisions: First, the common chamber, S.c, 

 out of which the three semicircular canals are to be differentiated. Second, a 

 slender canal, D.e., which one easily -identifies as the anlage of the ductus en- 

 dolymphaticus. It lies between the semicircular canal and the wall of the me- 

 dulla oblongata. Third, the long, curving, but not spiral cochlea, Coch. The com- 

 mon chamber formed by the union of these divisions is later subdivided to form 

 the upper utriculus and lower sacculus. Outside the cochlea lies the cross-section 

 of the vena 'lateralis capitis, Jug, which appears in the adult as part of the inter- 

 nal jugular. Just below the lateral vein is the section of the motor portion, Fac.m, 

 of the facial nerve. The sensory portion of the facial nerve at this stage is 







