THE HEAD PROBLEM 



623 



neuromeres. While some hold that the segmentation visible in the open 

 neural plate is the true neural segmentation, others assume that the 

 secondary subdivisions of the brain vesicles are to be counted. 



Opinions differ also as to the criteria of neuromeres. Some observers 

 find the neural segments inconstant and asymmetrical. It is, therefore, 

 not surprising that the majority of those who have studied neuromeric 

 segmentation are skeptical of their metameric value, those of the hindbrain 

 possibly excepted. If any brain divisions anterior to those of the hind- 

 brain may be counted as neuromeres, they are the primary forebrain and 



Fig. 517. — Diagram of the segments (neuromeres, myotomes, etc.) of the head 

 A, anterior myotome; a, abducens nerve; b, branchial clefts; /, facial nerve; g, glosso- 

 pharyngeal nerve; h, hypoglossal nerve; I, lens, surrounded by layers of eye; n, nasal 

 pit, near it the terminalis nerve; o, oculomotor nerve; op^, ophthalmicus profundus part 

 of fifth nerve; os^, ophthalmicus super facialis part of fifth nerve; ot, otocyst; s, spiracular 

 cleft; t, trigeminal nerve; ta, truncus arteriosus; tr, trochlearis nerve; I-VIII, neuromeres; 

 1-6, myotomes. (From Kingsley's "Comparative Anatomy of Vertebrates," after 

 Neal.) 



midbrain vesicles, which owe their primary appearance to their functional 

 association with the olfactory organs and the paired eyes. It is possibly 

 significant that these two vesicles, like the neuromeres of the hindbrain 

 show a numerical correspondence with mesodermal somites, including 

 Miss Piatt's anterior somites. If this numerical correspondence seen 

 in elasmobranch embryos is not wholly accidental, the neuromeres, hke 

 the mesodermal somites, may have a metameric value. (Fig. 517) 



In 1885 Beard and Froriep independently discovered that the ganglia- 

 of the cranial nerves receive cellular accessions from epidermal placodes 

 above the gill-pouches. To these placodes Beard gave the name epi- 

 branchial sense-organs. Since they may be assumed to have a primary 

 numerical correspondence with gills, they were used by Beard as criteria 

 of the primitive segmentation of the head. (Fig. 518) 



