NERVOUS SYSTEM OF A TWO-HEADED PIG EMBRYO 409 



but rather, masses of tangled nerve fibers and cells of uncertain 

 significance, attached to the brain wall by short strands of nerve 

 fibers (figs. 5, 9 to 12). In contrast, therefore, these are desig- 

 nated cranial nerve-masses which are spatially separated into 

 two morphologically distinct aggregations, the one almond 

 shaped and rostral to the other which is a longitudinally elongated 

 mass. The former is designated as the rostral cranial nerve- 

 mass (figs. 5, 6, 9, 10, 11), the latter as the caudal cranial nerve- 

 mass (figs. 5, 6, 12 to 14). Each receives a definite set of nerve 

 roots. The topographical position of these two cranial nerve- 

 masses gives a clue that is confirmed by tracing the median nerves 

 brainward from their peripheral distributions, in so far as this 

 is possible. It should be mentioned in passing that there are 

 fiber strands scattered through this median region with no peri- 

 pheral or proximal connections and therefore impossible of 

 accurate identification. By the brainward tracing of the identifi- 

 able nerves, the rostral cranial nerve-mass is apparently a 5-G-7-8 

 nerve complex, and the other a 9-10-11-12 complex (fig. 5). 



A median (conjoined) gasserian ganglion is a conspicuous 

 neurological landmark of certain identification in the fused area 

 of the two heads (figs. 5, 10). It is small as compared with the 

 gasserian of either normal side, but large for the limited space 

 of the median area. From it an ophthalmic division with a 

 conspicuous nasociliary branch is given off to the inner side of 

 each head. Other small branches are also given off, but lose 

 themselves in near-by tissue without yielding assurance of 

 identity. The ganglion connects with the rostral cranial nerve- 

 mass by a median compact strand of fibers which could not be 

 traced through it with certainty (fig. 5). To one side of the 

 rostral cranial nerve-mass, and lying between it and nerves three 

 and four of the right (inner) side of head B, a short, stout branch 

 from the ganglion ends abruptly in connective tissue and its 

 identification is therefore vague (fig. 5, 9) . 



Above the ganglion there is a pair of detached, unidentifiable 

 nerves on either side of the midline, which may well belong to 

 the median trigeminus, since the fifth nerve has such an extensive 

 distribution, but again there is no assurance of accurate identity. 



