1 90 Journal of Comparatwe Neurology and Psychology. 



segments and the somites is prevented in Petromyzon by the 

 thickness of the gut wall in early stages. (It is possible that in 

 ancestral vertebrates with less yolk a complete correspondence 

 might have occurred.) With the using of the yolk and thin- 

 ning out of the gut wall a numerical correspondence becomes 

 possible, and it is important that the gill apparatus never ex- 

 tends beyond the point at which the first subbranchial muscle 

 bud is to form ; — the point, that is to say, which should mark 

 the caudal limit of the gills on the theory of complete corre- 

 spondence of the two segmentations. 



The investigation of other cyclostomes should throw further 

 light on this important question. For the present, the many- 

 sided indirect evidence which comes to impress oi,ie who at- 

 tempts to frame a consistent account of head segmentation as a 

 whole impel the writer to consider that the visceral apparatus 

 and the dorsal mesoderm have a common segmentation. In 

 accordance with this view the following statements may express 

 the most probable conclusions regarding branchial segmentation 

 and musculature. 



a. There is essential correspondence between branchiom- 

 erism and mesomerism, the mandibular arch corresponding to 

 somite 2. 



b. This correspondence probably rests on a primitive com- 

 plete segmentation of dorsal and lateral mesoderm, tlie gill slits 

 being formed in the clefts between the mesodermic segments. 

 Such a condition persists in the case of the mandibular and 

 hyoid arches and the first and second visceral sacs in some ver- 

 tebrates. 



c. The lateral mesoderm forms visceral muscles moving 

 the gill arches and the lower jaw. The nerves supplying mus- 

 cles derived from the lateral mesoderm arise from the lateral 

 motor nucleus in the medulla oblongata and are properly known 

 as viscero-motor nerves. 



d. Similarly the trapezius group of muscles are derived 

 from the lateral mesoderm and are innervated by viscero-motor 

 nerves. On this ground the pectoral girdle is perhaps to be re- 

 garded as a modified branchial arch. This is further supported 



