FRONTAL SECTIONS OF EMBRYO OF J2 MM, 



213 



attenuated deck-plate. The trigeminal ganglion, G. tri, is very large and sends 

 its sensory fibers upward into the dorsal zone to form there a distinct bundle of 

 nerve-fibers which persists throughout life and is known in the adult as the 

 tractus solitarius, T. S. The other root of the nerve, L. R, is lateral. It lies 

 below the ganglion near the median plane. Its fibers arise from neuroblasts in 

 the ventral zone and gather together as a distinct bundle which starts near the 

 median line, takes a curving course 

 through the ventral zone, and makes 

 its exit from the medullary wall at the 

 dorsal limit of the zone. It has a strik- 

 ing resemblance to the root of the facial 

 nerve. We do not yet know whether 

 such a course of the fibers is character- 

 istic of all lateral roots or only of the 

 trigeminal and facial roots. On the 

 medial side of the trigeminal ganglion 

 is a large vein. Card, the anterior car- 

 dinal vein, which by island formation is 

 to migrate to the outside of the gan- 

 glion to form a portion of the permanent 

 jugular trunk. In the median line in 

 the mesenchyma immediately below 

 the raphe is the section of the basilar 

 artery, and considerably below that is 

 the small section of the notochord 

 which it is very difficult to distinguish 

 with a low power. Between the noto- 

 chord and the cardinal vein is the sec- 

 tion of the carotid artery. 



Section through the A custico- facial 

 Ganglion. — In this section (Fig. 126) 

 the thickened ventral wall of the hind- 

 brain (i. e:, the anlage of the medulla 

 oblongata) is not spread out nearly 

 horizontally, as in the trigeminal region, but rises obliquely on either side 

 from the median line. The right and left sides of the medulla are divided 

 from one another by a deep median fissure, /. In the median line we see also 

 the basilar artery, A. bas, and still lower the wide, slit-like pharynx, Ph, the 

 outer portion of which ascends obliquely toward the jugular vein, Jug. The 



Mx.i 



Fig. 126. — Pig, 12.0 mm. Frontal Series 6, 

 Section 340. 



A. bas, Arteria basilaris. D.Z, Dorsal zone of the 

 medulla oblongata. £c, Ectoderm. ^/^«, Epen- 

 dymal roof of the fourth ventricle. /, Median 

 fissure of the medulla oblongata. Fac, Sensory 

 root of the facial nerve. G.gen, Geniculate 

 ganglion of the facial nerve. G.vest, Ganglion 

 vestibuli of the acoustic nerve, /ttg. Jugular 

 vein, mes, Mesenchyma. Mx.i, Inferior max- 

 illary branch of the trigeminal nerve. N.12, 

 Hypoglossal nerve. Ph, Pharynx, t.m, Motor 

 tract of facial nerve. X 22 diams. 



