The Cliondrocraniiini of an Embryo Pig. 191 



then forward into the ganglion spheno-palatinum. From the hiatus, 

 the facial nerve curves outward and downward and enters the sulcus 

 facialis, which it follows backward through the tympanic cavity 

 (Figs. 3 and 4), passing over the stapes and medianly to Reichert's 

 cartilage. 



Vrolik says (c/. van Kampen, p. 388, note) that the sulcus facialis 

 forms a completely closed canal in Sus embryos. He must be mis- 

 taken in this, for in my reconstruction the sulcus is open for its full 

 length, and even in the adult animal only the anterior half forms a 

 closed channel. After crossing the proximal end of Reichert's car- 

 tilage, the facial nerve divides, the anterior branch going forward 

 as the chorda tympani, while the main stem, the motor portion of 

 the facial nerve, curves forward and follows the outer side of the 

 comu hyale to a point opposite the anterior end of the ear-capsule, 

 where it divides into a number of small branches. The chorda 

 tympani (Figs. 3, 4, 5 and 8,c/^. ty.), after leaving the facial nerve, 

 swings around to the lateral side of Reichert's cartilage, passes be- 

 tween the manubrium mallei and the crus longum incudis, and con- 

 tinues forward on the median side of Meckel's cartilage to where 

 it joins the ramus lingualis, nervi trigemini, in the submaxillary 

 ganglion. At this stage of the embryo, the tympanic cavity has not 

 attained its final large size, so that the chorda tympani does not run 

 through the cavity proper but through the adjacent tissues, which are 

 later absorbed as the cavity enlarges. 



In the postero-lateral wall of the pars cochlearis is a large open- 

 ing, the fenestra cochleae or rotunda (PL III). Posteriorly it 

 communicates by a broad opening with the foramen jugulare, while 

 laterally it opens into the space later occupied by the tympanic 

 cavity. In its relations to the surrounding parts, it is almost identical 

 with the fenestra cochleae of Lacerta, if one considers that the an- 

 terior end of the fissura metotica (recessus scales tympani) has been 

 closed. 



Comparing the region around the tympanic cavity in the embryo 

 with the same in the adult, we notice in the 30 mm. embryo a com- 

 plete absence of all the parts later represented by membrane bones, 

 e. g., the processus anterior mallei, the squamosum and the osseous 



