FRONTAL SECTIONS OF EMBRYO OF 12 MM. 211 



quickly identified because it is the first behind the otocyst. The upper ganglion 

 of this nerve, the so-called Ehrenritter's ganglion, is represented by an accumula- 

 tion of cells in the upper part of this root. As regards the tenth nerve, or vagus, 

 both its ganglia and the fibrous trunk connecting them are shown. The upper 

 or jugular ganglion, G. jug, is nearly on a level with the otocyst, while the lower 

 or nodosal, G. nod, lies near the cervical sinus. To the nerve-trunk between 

 the two ganglia are adjoined the fibers of the eleventh or spinal accessory nerve, 

 which does not otherwise appear in this section. A small piece only of the hypo- 

 glossal nerve can be seen, A^. 12. The space occupied by this nerve is blank in 

 the engraving; in the specimen it shows horizontal fibers. 



The Study of Frontal Sections, Embryo of 12 mm. 



The frontal series has special value for the study of the hind-brain and asso- 

 ciated structures, as the plane of the section is approximately at right angles to 

 the axis of the hind-brain. It also furnishes instructive pictures of the relations 

 of developing vertebrae and nerves. 



Portions of three sections illustrating the structure of the hind-brain and 

 associated parts are given below. The following remarks on the hind-brain are 

 intended to make the significance of these sections clearer. The wall of the hind- 

 brain is, of course, produced by the development of the wall of the medullary 

 tube. Its most striking peculiarity is the enormous expansion of the deck- 

 plate, which forms the very wide epithelial layer, epen, the so-called ependymal 

 roof of the fourth ventricle. It starts from the upper edge of the dorsal zone. 

 Fig. 125, D. Z, and forms a wide arch which is covered in externally by a rather 

 thin layer of mesoderm, mes, and the nearby epidermis, Ec, of the embryo. The 

 covering is so slight in development at this stage that in the fresh specimen the 

 roof of the fourth ventricle, including its coverings, appears as a translucent mem- 

 brane through which we can readily distinguish the great cavity of the fourth ven- 

 tricle itself. The expanse of the ependymal arch is greatest at the region of the 

 trigeminal root. From there backward toward the spinal cord its expanse gradu- 

 ally diminishes. In correspondence with the growth of the deck-plate the lateral 

 walls of the medullary tube become bent outward and downward, so that, though 

 they remain near together on their ventral side, where they are united by the floor- 

 plate or median raphe (Fig. 127, raph), yet their upper dorsal edges are far apart. 

 In consequence of this change of their position the original lateral walls appear as 

 the floor of the hind-brain, and we recognize in them the anlages of the medulla 

 oblongata. We distinguish here, as everywhere in the medullary wall, the dorsal 

 and ventral zones. The ventral zone is intimately united with its fellow by the 

 short median raphe. Between them is a deep fissure (Fig. 1 26, /), which is never 



