254 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



parts of the original segments. Fortunately, however, with the knowl- 

 edge that neuromeres and mesomeres correspond numerically, we are 

 able to see that the majority of changes which have occurred are cor- 

 related ones, and thei'efore capable of explanation. We furthermore see 

 that the greatest changes have taken place in the more anterior meta- 

 meres, chiefly and primarily by tlie loss of the ventral parts of these 

 metameres. Since the more posterior of the cephalic segments have 

 indubitable metameric value, I shall discuss in detail only those anterior 

 ones (viz. I to VII) concerning which there is most disagreement among 

 morphologists, beginning with the consideration of the seventh, whose 

 relations are least modified. 



VIII. Primitive Relations of Cephalic Segments. 



a. Relations of Encephalomere VII. 



Opposite the posterior constriction of this encephalomere in very early 

 stao-es lies van Wijhe's 6th somite, which develops embryonic muscle 

 fibres and is universally considered a true somite. I therefore regard 

 this as the mesomere corresponding with encephalomere VII, whose 

 neural-crest cells first meet the mesoderm opposite the anterior constric- 

 tion of this somite (Plate 3, Fig. 13). These cells form the Anlage of 

 the anterior branch of the vagus (Urvagus), and I assume that the 

 primitive relations of this nerve were with the myoseptum between the 

 5th and 6th somites. The intermediate position of the Urvagus with 

 respect to the myotomes and its ontogenetic union with spinal ganglia 

 in some Vertebrates serves to show that there is no fundamental 

 difference in this respect between cranial and spinal nerves. For rea- 

 sons which will be stated in connection with the study of the relations 

 of encephalomere IV, I regard the abducens (Plate 4, Fig. 21), whose 

 fibres have their exit from the ventral horn of encephalomere VII, 

 as representing in part the ventral nerve of this segment. Furthermore, 

 I assume that the mesoderm of the 6th somite was primitively connected 

 with the mesoderm of the 4th visceral arch (Plate 3, Fig. 16); because 

 that somite in Ammoccetes which I regard as its exact homologue, viz. 

 the 2d post-otic somite, is certainly in early stages thus connected. 

 Consequently the present 3d visceral cleft bounds ventrally the visceral 

 (splanchnic) portion of this segment. 



