NERVOUS SYSTEM OF A TWO-HEADED PIG EMBRYO 385 



the head of the embryo treated as a unit. Considering the organ- 

 ism as a single symmetrical solid, the term median is used with 

 reference to the brain structures between the raphes of the two 

 heads, or to structures outside of the brain that lie in the area 

 between the diverged snouts. Inner side and outer side refer to 

 the two sides of each head, e.g., the 'outer side of head A' would 

 be its right side, the 'inner side of head A' its left side. Normal 

 sides refer to . the outer sides of the heads in contradistinction to 

 the median area. Rostral structures are those toward the snout 

 end of the organism; caudal (or caudad) toward the tail. Con- 

 joined structures are those containing contributions from both 

 members. Dorsal and ventral refer to the back and ventral (belly) 

 sides, respectively. In the description of the nervous system the 

 sulcus limitans is taken as the boundary between the dorsal and 

 ventral quadrants. 



Among the more important morphological points to be kept 

 in mind are the embryological flexures, which are at their maxi- 

 mum, the axes and planes of the teras as a unit, and the normal 

 morphological axes and planes of each head. For example, there 

 are in reality three median sagittal planes, one for the embryo as 

 a whole and one for each head. To avoid confusion, the latter 

 is referred to as the 'median plane of head A' or 'median plane of 

 head B,' whereas 'median plane' is restricted to the median 

 sagittal plane of the monster as a whole. 



The median plane of the monster represents juxtaposed exter- 

 nal surfaces of the two heads, i.e., the left side of head A is approxi- 

 mated to the right side of head B. This relationship is of great 

 importance in interpreting the structures in the median region 

 of the organism, the region of greatest interest. 



Because head A is tilted downward slightly more than head 

 B, and the top of head B consequently appears higher, the plane 

 of a cross-section in a given region does not always pass through 

 the same structures in the two heads. This gives a false impres- 

 sion of asymmetry. For example, owing to this tilting, the 

 outer eye of head A lies somewhat caudad to that of head B and 

 a section cutting through the middle of the lens of B passes just 

 rostral to the lens of the outer eye of head A (figs. 14, 15). Sec- 



