TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 177 



left side (the right in the figure); the ganglion is a small, dark mass of triangu- 

 lar outline close to the medullary wall of the hind-brain, and it lies almost even 

 with the posterior edge of the otocyst. A r . 10 is the vagus nerve (the section passes 

 through the upper portion of the jugular ganglion of that nerve) ; it shows better 

 on the right side of the embryo in this section than on the left. Com. 10, and com, 

 refer to the ganglionic commissure which extends above the origins of the hypo- 

 glossal nerve. On the left the continuity of this commissure is better shown 

 than on the right, where it offers two parts, one, com, entirely ganglionic, and 

 another, com. 10, which comprises both the ganglionic portion and fibers which 

 share in the formation of the root of the tenth nerve. The trigeminal ganglion 

 is very large, somewhat triangular in shape, the apex of the triangle joining the 

 angle of the hind-brain. This situation is very characteristic, for the trigeminal 

 is always at this angle, and, from its great size and position, is one of the most 

 important landmarks in the study of the topography of the embryonic head. 

 The nerve-cells of the ganglion are grouped, for the most part, on the side toward 

 the ectoderm, where they are closely crowded together, making a deeply staining 

 mass. Nearer the brain-wall the tissue of the ganglion is much less condensed, 

 is somewhat penetrated by small blood-vessels, and contains a considerable 

 number of nerve-fibers, which are gathered into small bundles. Toward the 

 brain-wall the bundles become distinct, and on the right side of the embryo the 

 passage of the nerve-fibers into the brain can be readily seen. The nerve-fibers 

 at this stage are merely neuraxons, that is to say, merely thread-like prolonga- 

 tions of the bodies of the nerve-cells (neurones). The fibers are entirely without 

 sheaths. They stain very lightly, and hence, in the preparation, may be 

 detected by their light appearance. The nerve-fibers may be conveniently ren- 

 dered conspicuous by counterstaining the sections with Lyons blue. The nerve- 

 fibers of the trigeminus, which enter the wall of the hind-brain, form in part a 

 bundle of fibers, which extends along past the acustico-facial ganglia within the 

 medullary wall. These fibers represent the commencement of the ascending 

 trigeminal tract of anatomists. The other ganglia associated with the hind-brain 

 are not well shown in this section. The otocyst has a very sharply defined epi- 

 thelial wall and is imbedded in loose mesenchymal tissue. On the right side of 

 the embryo we have the ductus endolymphaticus, D.E, the opening of which into 

 the main cavity of the otocyst is shown on the left side. The epithelial wall of 

 the ductus is thicker than that of the greater part of the otocyst proper. The 

 wall, Md, of the hind-brain exhibits already characteristic differentiations, for 

 it shows clearly the three primitive layers ; the outer or external neuroglia layer 

 is thin, and appears light in the section because it takes the stain slightly. It is 

 in this outer neuroglia layer (ectoglia) that the entire sensory nerve-fibers are 



