206 BULLETIN: MUSEUM OF COMPARATIVE 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.! 
Y 
Cer Q 
he 
07 
Fieurn 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 Platt should be ho- 
mologized with the “head cavities” (vordere Entodermtasche) 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 myocal from the median wall of the cavity. 
1, 2, first and second head cavities; a, “anterior cavity” (Platt); ec'drm., 
ectoderm; vs. opt., optic vesicle. 
