NERVOUS SYSTEM OF A TWO-HEADED PIG EMBRYO 411 



caudal cranial nerve-mass is separated by a short hiatus from the 

 rostral end of the median spinals (fig. 5). 



The median isolated ganglion 



Wilder ('08, p. 419) speaks of this ganghon as follows: "Poste- 

 rior to this" (i.e., a long ganghonic ridge extending longitudinally 

 in the midventral hne posterior to the sixth nerve) "and still in 

 the median Hne are two farther nerves which may be regarded 

 as cranial, the first smaller and the second large" (the probable 

 ninth and tenth). "About opposite these ventrally lies a large 

 median ganghon, entirely detached from them and from the 

 cord, yet from this proceed two pairs of nerves, the anterior 

 ones very fine and short, the posterior ones larger and longer. 

 These nerves seem soon to lose themselves in the surrounding 

 tissues." He then identifies a glossopharyngeal and vagus, and 

 adds: ' 'The detached ganglion is less certain" (than the probable 

 9th and 10th) "at least until after a careful determination of 

 the rudimentary structures to which its nerves are distributed, 

 but it is probably either the ganghon petrosum of the ninth or 

 the ganghon nodosum of the tenth, or possibly both together." 



From the evidence at hand I feel obliged to disagree with 

 Doctor Wilder 's interpretation, although the ganghon lies in the 

 plane of the median 'vagus complex' (the caudal cranial nerve- 

 mass of this paper). The anatomical factors that support the 

 difference of opinion are the identifications of certain structural 

 landmarks in this median region, which has been referred to 

 several times as a region of special interest. Until these were 

 identified the ganghon remained the neurological conundrum 

 of the embryo. It seems advisable, therefore, to digress long 

 enough to estabhsh these landmarks before hazarding an inter- 

 pretation of the ganghon. The several reconstruction models 

 were invaluable in this connection. 



The only external landmark to be noted is a shght depression 

 in the midhne of the teras below the median eyes and between 

 the inner angles of the mouths. This is a median (conjoined) 

 external auditory meatus (figs. 1, 2, 19). Studying first in the 

 sections the normal (outer) sides of heads A and B, the junction 



