NERVOUS SYSTEM OF A TAVO-HEADED PIG EMBRYO 417 



midline itself is therefore the line of union between the external 

 (peripheral) surfaces of the dorsal quadrants (fig. 9). On either 

 side of the midhne, but very close to it, is a fiber tract, which by 

 analogy with the tracts in similar position on the normal sides 

 of the teratological head is beheved to be a spinal fifth tract of 

 the left and right sides of the two heads, respectively (fig. 9). 

 In size each tract is greatly reduced, proportionate to the reduced 

 size of the quadrants, but in extent quite in keeping with the 

 elongated, normal tractus spinahs. In shape it conforms to the 

 changing configuration of the quadrants, and rather far rostrally 

 it curves crescent wise into the ventral quadrant, which, however, 

 lies lateral to the dorsal quadrant. 



Bundles of nerve fibers from the cranial nerve-mass cross the 

 ventral end of the median spinal fifth tract and turn spinalward 

 at a sharp angle to keep within the dorsal quadrant, as it were, 

 only to lose themselves in the dense cells of the mantle layer. 

 These lie laterodorsal to the spinal fifth tract, but represent a 

 median position in normal morphology. They suggest sensory 

 fibers of the fifth nerve (fig. 9). Other fibers cross the ventral 

 end of the tract and take a more lateral direction toward the 

 ventral quadrant, but since the line of demarcation between the 

 two quadrants is an arbitrary one drawn from the sulcus limitans 

 to the periphery, it is exceedingly difficult to determine just 

 where the fibers end, since they are obscured by the cells of the 

 mantle layer. Although some of them may be motor fibers of 

 the fifth nerve, I believe most of them to be sensory fibers of the 

 trigeminus, for this is the chief nerve involved at this level. 



VI. Nothing definite could be estabUshed for the median 

 abducens fibers. 



VII. The intracerebral course of the median pair of facials is 

 distinctly traceable from the genu internum to the cranial nerve- 

 mass, but here they interlace in such manner as to render further 

 identification impossible (figs. 10, 11). 



VIII. From the caudal area of this rostral cranial nerve-mass, 

 but teratologically medial to the seventh nerves (i.e., normally 

 dorsal), a bundle of nerve fibers enters the brain on either side 

 of the midline of the monster and is faintly traceable in the 



