PLATE 2 
EXPLANATION OF FIGURES 
Figures 11 to 17 are drawn from median sagittal sections of Squalus embryos 
of different stages showing the limits and mutual relations of the ectoderm and 
endoderm in the region of the mouth and pharynx. ‘The visceral pouches and 
clefts were superimposed from in toto preparations. Endodermal structures are 
cross-hatched and nervous system stippled in all drawings. 
lland12 Median longitudinal sections of Squalus embryos in Balfour’s stages 
G and H, showing the relations of the endoderm and ectoderm before the per- 
foration of the buccal plate. 
13 A median longitudinal section of stage I following the partial perforation 
of the buccal plate. The deeply staining properties of the ectodermal cells and 
the coarse yolk granules in the endodermal cells make easy the determination 
of the boundaries between ectoderm and endoderm at this stage. Two visceral 
clefts and two pouches formed. 
14 In the region of the hypophysial invagination a remnant of the double- 
layered buccal plate remains as the so-called preoral lobe. There are three vis- 
ceral clefts and two pouches. Stage K. 
15 Stage L. The hypophysis still retains in connection with the ectodermal 
stomodaeum. The pharynx still retains its endodermal lining. The ectodermal 
ingrowth shght. 
16 A median longitudinal section of a 60-mm. embryo in which taste papillae 
and buds are distinguishable in both floor and roof of the pharynx. The ecto- 
dermal ingrowth does not extend farther than the limits of the upper and lower 
jaws. 
17 A more enlarged drawing to show the relations of ectoderm and endoderm 
in the region of upper and lower jaws. The taste-buds are limited to the epi- 
dermal surfaces formed from endoderm. 
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