200 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
orale Darm ”), while its walls become continuous anteriorly with that 
mass of tissue which later differentiates into the “anterior cavities.” 
Furthermore, a cross section of a corresponding stage of development in 
a plane immediately posterior to the infundibulum (i, e. along the line 
a B of Figure C) gives equally convincing evidence (shown in Figure D) 
that the mass of cells (1) lies dorsal to the wall of the alimentary canal, 
with which, however, they are in close connection in this-somewhat earlier 
stage (11-12 somites). There exists not the faintest shadow of evidence 
that the mass of cells which forms in its lateral part the premandibular 
cavities and in its median part their connecting stalk, represents entoder- 
Figure Č. 
mal diverticula, During development, as the result of the ventral growth 
of the infundibulum, the pre-oral (Seessel’s) pouch becomes obliterated and 
the mass of cells surrounding the “anterior cavity” is cut off from those 
posterior to the infundibulum (26-27 somites). By this change in rela- 
tions the Anlagen of the connecting stalk and of the premandibular cavity 
take a position apparently anterior to the alimentary canal and in close 
Fic. ©. Median sagittal section of a Squalus embryo with 14 or 15 somites. 
The neural folds have not as yet met in the mid-dorsal line. X 77. The meso- 
derm of the connecting stalk of van Wijhe’s first somite is seen as a thickened 
mass of cells lying between the base of the brain and the dorsal wall of the alimen- 
tary tract. 
1, mesoderm which later becomes differentiated as the connecting stalk of the first 
head cavity. a, mesoderm of the “ anterior cavity ” (Platt); arent., pre-oral pouch 
of the archenteron ; ent., dorsal wall of alimentary canal; ifb., infundibulum ; tb. n., 
ventral wall of the neural tube; a B, projection of plane of the section shown in 
Figure D. 
