252 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



A study of ueural segments anterior and posterior to the medulla has 

 led me to the conclusion that the local thickening is a more essential 

 characteristic of a hiudbrain neuromere than the commonly accepted 

 criteria, viz. the radial arrangement of cells in the neuromere, and the 

 crowding of them in the regions of constriction between neuromeres, 

 both of which may be the result of mechanical influences. 



The shifting of the point of exit of the roots primitively related to 

 encephalomeres VI and VII may easily be explained as the result of the 

 crowding caused by the ear capsule. Since four hindbrain neuromeres 

 are clearly related to four visceral arches, we should expect the remain- 

 ing one, encephalomere IV, to have been primitively related to a vis- 

 ceral arch. That such an arch has been present in the region of this 

 neuromere during phylogeny, has been made probable by the studies of 

 van Wijhe ('82), Miss Piatt ('91), and Hoffmann ('94). The evidences 

 from the study of neuromerism and mesomerism ai'e mutuall}' confirma- 

 tory, and to the effect that a visceral arch has been lost in the region of 

 encephalomere IV and van Wijhe' s third somite. Having established an 

 exact numerical correspondence between encephalomeres and s'omites 

 (head cavities), and a probable primitive correspondence of hindbrain 

 enceplialomeres with visceral arches, I conclude that in the head region 

 there existed primitively a correspondence between neuromerism.^ mesom- 

 rism, and branchiomerism. Since this correspondence is not to-day 

 exact in Squalus or in any other known Vertebrate, it seems necessary 

 to discuss somewhat in detail the constituent parts of the anterior or 

 more highly modified metameres, and to inquire what may be inferred as 

 to their previous conditions. The table on the opposite page, although 

 in part theoretical, will help to make the discussion clearer. 



I have in this table included neuromeres as far posteriorly as the 

 eleventh. Accepting Hoffmann's ('94) conclusion that vertebral arches 

 as far back as that which corresponds with van Wijhe's tenth somite 

 fuse into the cranium of the adult Squalus,^ it would, follow that neuro- 

 meres I to XI would be included in the cranium. The variability 

 in the number of segments added to the occipital region of the cra- 

 nium in different Selachii and Ichthyopsida (Fiirbringer, Sewertzoff) 

 makes the exact number in Squalus a matter of no gi-eat morphological 

 importance. 



We see that the cephalic segments are highly modified segments 

 altered by reduction or enlargement (possibly even by substitution and 

 change of relation, as, for example, in the case of the vagus segments) of 

 1 Recently confirmed by Sewertzoff ('98). 



