1 86 Journal of Comparative Neurology and Psychology. 



brates the mesoderm is completely segmented transversely at 

 an early stage and that the dorsal part (somite) is not clearly 

 separated from the lateral (branchiomere) at any time. This 

 is always true of somite 2 and the mandibular arch. In sela- 

 chians the transverse segmentation precedes the separation of 

 somite 3 from the hyoid arch. Even in amphibia (Amblystoma)^ 

 the hyoid arch is connected with somite 3 and is separately 

 split off from the entoderm, having had no connection with the 

 following lateral mesoderm. Further, the anterior head somites 

 develop as gut pouches in cyclostomes in reminiscence of the 

 mode of mesoderm formation in Amphioxus. The late con- 

 nection of somite i with the entoderm is a slight indication of 

 the same thing in selachians (57, 17). In all these respects 

 the head mesoderm presents primitive conditions. Now if it 

 be thought that one or more gill slits and arches have ever 

 existed in front of the present mouth, it is in no way unrea- 

 sonable to suppose that some vestige of the branchial arches 

 (lateral mesoderm) may be retained indistinguishably connected 

 with somite i and the anterior head cavity, just as the mandib- 

 ular arch is attached to somite 2 without definite boundary 

 line. The fate of such lateral mesoderm other than mesen- 

 chyme formation is not known. 



f. Somite 4 lies mesial to or slightly caudal to the audi- 

 tory vesicle during its formation and immediately in front of 

 the ganglion of the glossopharyngeus. This is consequently 

 often designated as the first postauditory somite. 



g. More or fewer of the postauditory somites become re- 

 duced or disappear during development. Petromyzon forms 

 an exception to this and the conditions there as described by 

 KoLTZOFF {76) are sufficiently important to be reported in this 

 connection. Somites 4 and 5 become divided into mesial and 

 lateral parts which are completely separated in somite 4, but in 

 somite 5 remain connected by a slender bridge. The mesial 

 part of both of these somites lies close to the chorda and later 



' References to Amblystoma are based on my own observations not before 

 published. 



