NERVOUS SYSTEM OF A TWO-HEADED PIG EMBRYO 421 



There is a pair of well-developed median eyes in a single palpebral 

 opening (figs. 1, 2, 10). A median conjoined auditory meatus 

 is present (figs. 1, 2, 19). Eyes and ears on the outer sides are 

 normal. 



2. All of the tissues appear to have been in healthy condition 

 when the embryo was killed. 



3. Duplicity is more evident internally than externally, but 

 bilaterality is strikingly apparent in all of the structures of the 

 doubled region. 



4. In the development of the teras the primitive morphological 

 relationships and the normal embryonic flexures have remained 

 undisturbed. The topographical changes in position of certain 

 structures in the conjoined (median) region are apparently 

 merely secondary, regulatory adjustments. 



5. The morphological and teratological planes and axes are 

 not necessarily identical. 



6. The nervous system is well developed and proportionate to 

 the size of the embryo. 



7. The spinal cord is incompletely double and gives off laterally 

 paired spinal nerves in normal manner (figs. 7, 20). Along the 

 median ventral surface of the cervical cord a neural ridge gives 

 off a short series of median (conjoined) spinal nerves, ungangUon- 

 ated and unbranched (figs. 5, 20). 



8. The cervical sympathetic system is well developed on the 

 normal sides. As in normal embryos, it consists of a stout 

 truncus sympatheticus which parallels the trunk of the vagus 

 nerve, curves dorsalward of the ganglion nodosum, and terminates 

 in connective tissue medial to the petrosal ganghon of the glos- 

 sopharyngeus (fig. 7). In addition, a shorter truncus between 

 it and the normal spinal nerves connects it with them (fig. 7). 

 In the median region the spinal sympathetic ganglia are poorly 

 developed, being hmited to a small patch of cells at the terminal 

 end of the growing nerve trunks of the last two median spinal 

 nerves (fig. 20, s. 362). The cranial sympathetic gangha are 

 present on the normal sides of the head members (figs. 15, 16, 

 17, 19). In the median region only the sphenopalatine gangha 

 were identified with certainty (figs. 13, 14). Ciliary and otic 



