200 



BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



orale Darm"), while its walls become coutinuous 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 /8 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- 



^<2' 



Figure C. 



mal diverticula. During development, as the result of the ventral gi-owth 

 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 



Fig. C. 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 Wijlie'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 " (Piatt) ; ar'ent., pre-oral pouch 

 of thearchenteron; enf., dorsal wall of alimentary canal; t|/7(., infundibulum ; tb. jk, 

 ventral wall of the neural tube ; a P, projection of plane of the section shown in 

 Figure D. 



