SAGITTAL SECTIONS OF EMBRYO OF 12 MM. 295 



dark rounded mass, the anlage of the nodulus thymicus (compare Fig. 194, Nod}. 

 The nodulus anlage is produced by proliferation of the entodermal cells on the 

 anterior side of the third cleft, and is penetrated by blood-vessels which seem to 

 be sinusoids, although their history has not been worked out. The great vein of 

 the head, which for convenience we may term the jugular, although the applica- 

 tion of this name to the vein in its present condition is somewhat inexact, is cut 

 several times, owing to its irregular course. Its main stem, Jug"" , arises nearly 

 vertically through the cervical region and is, relatively to the size of the embryo, 

 of huge diameter. It continues upward, Jug'", along the dorsal side of the vagus 

 to about half-way between the ganglion nodosum and ganglion jugulare. At that 

 point the vessel curves inward and forward, and therefore is not encountered 

 again in this section until, having bent upward again, it shows, Jug', on its way 

 past the trigeminal ganglion. A branch of the jugular, Jug", is cut just above the 

 ganglion, and another small and probably not very important branch is shown 

 at Ve. 



The nerves are shown as follows: The optic nerve, N.op, still has its central 

 cavity, which, nearer the median plane, opens into the third, ventricle of the brain, 

 and in the section resembles in shape an inverted U. On the side of the nerve 

 toward the mouth there is a deep notch the section of the choroid fissure. The 

 trigeminal ganglion, G.tri, .is very large, and its trilobate form is clearly indicated 

 by the figure. The lobe to which the reference line, G.tri, runs gives off the ramus 

 ophthalmicus; the lobe nearest the jugular gives off the ramus maxillaris inferior, 

 while the middle lobe gives off the ramus maxillaris superior. From the ganglion 

 the fibers and nerve-cells extend upward to form the root, AT. 5, which joins the 

 hind-brain at a characteristic point namely, at the summit of the Varolian bend 

 and where the hind-brain is widest (compare Figs. 189 and 203). By its great 

 size and by its topographical association with the lateral apex of the recessus lateralis 

 of the fourth ventricle, the trigeminal ganglion may always be readily identified 

 in sections of embryos. The acustico-facial ganglia, Ac.F, may also be readily 

 determined by their typical position immediately in front of the otocyst, Ot. But 

 it is quite difficult to identify the four components of this complex structure; 

 namely, i, the motor root of the facial nerve; 2, the facial or geniculate ganglion; 

 3, the vestibular ganglion; 4, the cochlear ganglion. In figure 202 three divisions 

 are shown. The large, darkly stained division, to which the reference line, Ac.F, 

 runs and which lies nearest to the otocyst, is the vestibular portion of the acous- 

 tic ganglion; the small, light area occupying a middle position in the inferior part 

 of the complex is the motor division of the seventh nerve, or lateral root of the 

 facial; it can be followed to the brain, which it enters as four bundles of fibers; 

 its path of entrance is shown better in frontal sections (Fig. 204, t.m). Just in 

 front of the facial motor root lies a second smaller dark mass, the geniculate gan- 

 glion of the facial, with an upward prolongation, the sensory root. The ninth or 

 glosso-pharyngeal nerve is represented by the ganglion petrosum, G.petr, and its 





