296 STUDY OF PIG EMBRYOS. 



ascending sensory root. This nerve may be 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 accumulation 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 ganglion, 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 hypoglossal nerve can be seen, N.I2. The 

 space occupied by this nerve is blank in the engraving; in the specimen it shows 

 horizontal fibers. 



Pig Embryo of 12.0 mm. Study of Frontal Sections. 



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 vena cava 

 inferior and of the relations of developing vertebrae and nerves. 



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

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

 tended to make clearer the significance of these sections. 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, (Fig. 203, epen), the so-called ependymal 

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

 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 membrane 

 through which we can readily distinguish the great cavity of the fourth ventricle 

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

 geminal root. From there backward toward the spinal cord its expanse gradually 

 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. 205, 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. 204. /) ,which is never 

 wholly obliterated. The floor-plate undergoes a great development in later stages 

 and is transformed into the median raphe of the adult medulla. The lateral or 



