NERVOUS SYSTEM OF A TWO-HEADED PIG EMBRYO 395 



clearly shown by comparing cross-sections at various levels; for 

 example, at the lowest level it is somewhat evaginated, forming 

 part of a cranial nerve-mass to be described later (figs. 16, 17), 

 but at a somewhat higher level, and for about half the caudo- 

 rostral thickness of the medulla, it is greatly narrowed laterally 

 and elongated dorsoventrally, and owing to its limited space, 

 projects tongue-like into the central canal, giving it the forked 

 or inverted-Y appearance as described (fig. 13). At the rostral 

 end, however, the medullary canal becomes incorporated into 

 the fourth ventricle, and as the prongs of the Y flatten out in 

 consequence thereof, the area between them narrows dorsoven- 

 trally and spreads laterally, thus contributing effectively^ to the 

 abnormal width of the brain in this region and to the divergence 

 of the two forebrains (figs. 6, 9 to 12). This lateral spreading is 

 first noticeable at about the level of the superficial origin of the 

 fifth pair of cranial nerves of the normal (outer) sides of the 

 embryo. From here rostrally the angle of divergence increases 

 steadily to the isthmi of the double brain. At this morphological 

 level the rostral portion of the conjoined rhombencephalon (i.e., 

 the metencephalon) abruptly narrows toward the median sagittal 

 plane of the embryo, and forms a mound of tissue elongated 

 dorsoventrally aftd lying between the separated caudal ends of 

 the mesencephala (figs. 4, 6, 8). 



A careful analysis of the brain tissue between the prongs of 

 the Y-shaped canal shows it to be the walls of the juxtaposed 

 sides of the two head members united in the median sagittal 

 plane of the monster, i.e., a conjoined medulla made up of the 

 fused dorsal and ventral quadrants of the left medullary moiety 

 of head A and of the right medullary moiety of head B. The 

 size and topography of these quadrants varies with the degree 

 of fusion, as already indicated (seemingly a purely regulatory 

 adjustment), but the interpretation of the area is, I believe, clari- 

 fied by the identification of nervous elements within it and by 

 other definite anatomical landmarks, such as the two sulci limi- 

 tantes and the two median raphes of the conjoined halves of the 

 two heads. The behavior of the quadrants in the mounded tis- 

 sue between the midbrains is best understood after a description 

 of the fourth ventricle, and is therefore deferred until then. 



