118 MR. E, S. GOODRICH OX THE OCCIPITAL REGION 



develop gills or take on the function of gill -formation. The 

 posterior limit of the skull is doubtless altered in the same way, 

 and the position of the occipital condyles may shift vip or down 

 the segmental series. There should, therefore, be no theoretical 

 objection to accepting the ana,tomical and embryological evidence 

 that the occipital region of the head in Amphibia contains only 

 three segments. If segments conld really disappear, leaving no 

 trace behind, it would be hopeless to attempt to homologise 

 segments in any two forms. 



There is another theoretical consideration which seems to have 

 led to the adoption of the view that the occipital region of the 

 Amphibia is not as simple as it appears. It is urged that if it 

 contained only three segments, the Amphibia would be more 

 primitive than the Fishes from which they have descended. The 

 possibility of the reduction in the number of occipital segments 

 has just been explained above ; but is there really any necessity 

 to assume that it has taken place ? From whatever Fishes the 

 Amphibia may have been derived, we may be sure it was not 

 from fvilly specialised Selachians. Fi-om palaeontology alone we 

 may hope to obtain definite evidence on this point ; until con- 

 trary evidence is brought forward, there is no necessity to assume 

 that the ancestors of the Amphibia had more than three differen- 

 tiated occipital segments. Of all the living fishes the Dipnoi are 

 those which most closely approach the Amphibia ; even in the 

 modern Ceratodus there is no occipital condyle, no distinct limit 

 between head and trunk. It is true that several trunk-segments 

 have here been more or less completely assimilated to the skull 

 (Sewertzofif, 10) ; bvit thei-e is no reason to believe that in the 

 remote common ancestor of the Dipnoi and Amphibia the dividing- 

 limit between the two regions could not have been developed 

 three segments behind the auditory capsule. This seems to be 

 in agreement with the conclusions of Agar (1), based on a study of 

 the development of Lepidosiren and Protopterus '*. 



The hypoglossus in the Amphibia and Amniota may certainly 

 be considered as homologous, although not necessarily composed 

 of the same segmental nerves. It is owing to the shortness of 

 the skull in the Amphibian that the hypoglossal roots come out 

 behind it. 



Summary. 



The chief contents of this paper may be summarised as follows. 

 Three occipital segments occur in tlie head of Amhlystomu. Of 

 the three somites developed in these, the fii-st forms no muscle 

 and disappears early ; the second forms a myotome divided into 

 dorsal and ventral portions, of which the former alone persists, 

 fusing with the myotome next behind. The myotome of the 

 third segment persists dorsally, that of the first trunk-segment 



* The occipital region in fossil Amphibia seems to have been formed as in the 

 modern species. Important evidence as to the assimilation of the hypoglossal 

 segments in primitive Amniotes may perhaps be gleaned from a careful investiga- 

 tion of early fossil Reptiles. For instance, it seems to me not improbable that in 

 FareiasaHVus the occipital region is still in nn intermediate condition. 



