206 



bulletin: museum of compakative zoology. 



dence would seem to tell in favor of the view that this mesoderm 

 segment, like the following ones, is to be regarded as of somatic 

 value.^ 



Figure E. 



d. Summary. 



Neuromeres and somites show an exact numerical correspondence 

 throughout the length of the embryo. The serial alternation of myelo- 

 meres and somites evinces the metamerism of the former, while the 

 exact numerical correspondence of the encephalomeres and head so- 

 mites appears equally convincing evidence of the metameric value of 

 the encephalomeres. The head somites in Squalus are homologous with 

 those described by van Wijhe ('82) for Scyllium and Pristiurus, and 

 there is yet another anterior to these, viz. the " anterior " somite first 



1 As the most anterior of the cavities of the Selachian embryo, it would seem 

 more probable that the anterior cavities described by Miss Piatt should be ho- 

 mologized with the "head cavities" (vordere Entoderratasche) of Amphioxus, 

 than that the next following, the premandibular, should be. 



Fig. E. a cross section through the "anterior cavity" (frontal section of 

 the embryo) in an embryo with 78 somites. X 240. To show the proliferation of 

 cells into the myocoel from the median wall of the cavity. 



' 1, 2, first and second head cavities; a, "anterior cavity" (Piatt); ec'drm., 

 ectoderm ; vs. opt., optic vesicle. 



